How can the White Sox fix right-hander Luis Castillo?

When the White Sox acquired Luis Castillo from the Seattle Mariners at the trade deadline, they weren’t necessarily expecting the three-time All-Star version of Castillo to suddenly reappear. They did, however, need something closer to that than what they got.

The appeal was relatively straightforward: even amid a down season in Seattle, Castillo had a lengthy track record of durability and success. For a White Sox rotation that desperately needed someone who could consistently work deeper into games, Castillo represented an opportunity to acquire a veteran starter without surrendering one of the organization’s premier prospects (depending on how you felt about Boston Smith, of course).

Nearly a month later, the results haven’t been what the White Sox envisioned.

Through his first five starts with Chicago, Castillo owns a 6.45 ERA and 1.66 WHIP in 22.1 innings. He has allowed 31 hits, including seven home runs, while striking out 20 batters. His seven-inning, 10-strikeout performance against the Reds remains the clear outlier – if you remove that start, Castillo has allowed 16 earned runs in 15.1 innings in his other four outings.

With October approaching, time is running out for the White Sox and Castillo to make changes to turn those results around. The good news is that it appears there are still things the team can do immediately to improve his results – but it requires an understanding of who Castillo is now in his career, rather than who he has been. Since the team is also committed to Castillo beyond this season, there are also opportunities to help Castillo reinvent himself a bit as he enters a new chapter of his career. This article will explore both the potential short-term and long-term opportunities.

A quick plug: I did discuss this topic in a little bit of detail on the latest episode of the Sox On 35th Podcast. This article has much more depth, but if you’re interested in the video version, tap the player below.

With that, let’s begin.

Refining his pitch arsenal

Just like through most of his career, Castillo has four distinct pitches: a four-seam fastball, sinker, slider, and changeup. The White Sox have taken an important first step with a simple change in pitch mix: throw your best pitches more often and your worst pitches less often.

By throwing a season-low number of sliders thrown in August, which has been Castillo’s worst pitch, the White Sox have picked off the easy part of this.

The difference between Castillo’s wOBA and xwOBA on his slider suggests there may be some bad batted-ball luck in his results. That being said, this is the first time in the StatCast era (since 2017) that the Whiff Rate on Castillo’s slider is below 30% on the season (27.8%), so clearly there are some differences in how hitters have seen the pitch this year (more to come on this). By decreasing its usage and increasing his changeup usage to be about the same, Castillo and the White Sox saw season-high Whiff Rates on both pitches in August. So, perhaps there is still some evening out that can occur for some of these batted ball events.

That being said, there is still an issue in how Castillo’s pitches interact with one another, especially at an advanced point in Castillo’s career – those four-seam fastball and sinker results should’ve also popped out at you. Looking at Castillo’s pitch-location tendencies, his attack has increasingly become a version of hard up, soft down. His four-seam fastball predominantly works at the top of the strike zone. His changeup naturally lives toward the bottom.

In the images below, the left shows Castillo’s fastball locations since coming to the White Sox. The right shows changeups.

Now, in theory, that’s a common and effective way to create vertical separation that almost any pitcher uses in today’s game. But, there is a natural lack of deception in this approach for Castillo specifically. With a low, almost sidearm arm slot, Castillo is more of a typical east/west pitcher (think Jonathan Cannon) than north/south pitcher (think Sean Burke and his curveball). As we will dive into later later, Castillo’s pitching like a north/south pitcher while each of his pitches moves within an east/west profile.

Said differently: each of his pitches stays basically on the same horizontal plane. So, if a pitch starts up in the zone, it’s not going to have the bottom drop out like a Sean Burke curveball – it’s going to stay up there, and hitters can respond accordingly.

For a while, that didn’t matter for Castillo – his fastball was well above league-average, and it was a relatively new approach for him. But, pair this with the fact that the fastball is hovering around league-average nowadays while still getting below-average induced vertical break (12.4″), and you can start to build the narrative of how hard it can begin to be to get swing-and-misses at the top of the zone.

In fact, the last two seasons have seen his lowest Whiff Rates on pitches at the top of the strike zone since 2022. They are also his two lowest fastball velocity seasons.

So, to summarize: a pitch appearing high is almost guaranteed to be hard, while a pitch tracking toward the bottom is far more likely to be an offspeed pitch. And now, Castillo doesn’t have the velocity to overcome some of the predictability in this approach.

Castillo’s sinker should help disrupt this pattern – and, during the best seasons of his career, it did. From 2019-2022, Castillo’s sinker and four-seam fastball usage were relatively similar to one another. In 2020 and 2021, they were nearly identical. But a jump in four-seam fastball usage in 2023 represented the change to how Castillo has attacked hitters recently, chasing more of that top-of-zone swing-and-miss rather than trying to get more ground balls. This has also been reflected in his ground ball rate, which is at a career-low 36.2% in 2026 and has hovered around 40% or lower since this change in 2023 (before 2023, his ground ball rate was anywhere from 47% to 58%).

And, as we just talked about: it’s not working right now.

So, throw more sinkers, right? Well, maybe, but not necessarily. The more pressing need is where these sinkers are being thrown. Instead of being a bottom-of-the-zone velocity option, too many of Castillo’s sinkers have also occupied the middle and upper portions of the zone, leaving him without enough velocity at the bottom of the strike zone to complicate a hitter’s decision-making process.

In the images below, the first shows Castillo’s sinker locations with the White Sox. The second shows 2023-2025. The last one shows 2017-2022. So, you can see the full morphing of this pitch up in the zone over time.

At the end of the day, Castillo is known for having pretty good control, so hitters have the ability to sit on certain pitches in certain counts and have a pretty good feeling that, based on location, they’re going to get the pitch that it looks like they’re getting.

That’s particularly problematic because Castillo’s changeup is still capable of being a legitimate weapon. Among his four primary pitches in 2026, the changeup has produced the lowest expected wOBA (xwOBA) against it at .293. The White Sox need to make it harder for hitters to identify when it’s coming, especially as they’re clearly showing that they want to throw it more often.

There isn’t enough time remaining in the regular season for Castillo to make drastic changes, but a change to the use of the sinker to spot it more often at the bottom part of the zone could go a long way. Like I said, that doesn’t necessarily mean throwing substantially more sinkers. But, they certainly should try and change the purpose of the pitch.

If hitters see something heading toward the bottom of the zone, they need to account for the possibility that a 95-plus mph sinker is coming rather than automatically gearing down for a changeup. That creates uncertainty without forcing Castillo to overhaul the pitches he already throws.

Is Castillo tipping his pitches?

Another element worth monitoring is Castillo’s release. There are noticeable differences in the release points of each of his pitches, which raises the possibility that hitters are receiving additional visual information before the ball gets deep into its trajectory. That doesn’t mean Castillo is definitively tipping pitches, but combined with the stark difference in where his hard and soft pitches are typically located, it could be creating another layer of predictability the White Sox should investigate.

With more than three degrees of difference in arm slot on some of these pitches – including a difference of nearly 7.5 degrees between his fastball and changeup – these would qualify as “meaningful” differences.

If it’s easier, here it is in a more visual format. Again, note the clear differences in release point, particularly with his fastball and changeup.

Now, the argument against this would be that these aren’t particularly far off from Castillo’s career norms from 2022-2025. So, if Castillo has survived with this before, perhaps it actually seems like more of a tip to hitters than it actually is in reality.

It’s still something worth considering, however, especially as it relates to that all-important fastball velocity I keep mentioning. During his peak years with Cincinnati, Castillo’s four-seamer and sinker routinely averaged around 96-98 mph. His 2026 four-seamer is averaging 95.3 mph, while his sinker is at 95.5.

It’s still plenty of velocity, but at a certain point, it becomes a question of whether some things Castillo has been able to get away with throughout his career are starting to shift as he enters a different point in his career.

Which is where we get into the long-term idea…

A longer-term “reinventing” of Castillo

The more interesting question comes after the season. Castillo turns 34 in December. The White Sox owe him $22.75 million next season, and there’s at least a possibility that his 2028 option becomes relevant depending on his workload. In other words, they’re invested in making this work.

That makes this winter the appropriate time to explore something more significant: adding a breaking ball with considerably more horizontal movement.

Castillo currently throws a gyro slider. His 2026 slider has generated only about 0.7 inches of glove-side horizontal break – this has been the pitch that Castillo has thrown throughout his career, and there’s nothing about it that is different this season. However, take into account all of the other things we’ve talked about here, and naturally, it becomes a question of whether continuing to do relatively the same thing is the best path forward for Castillo.

Castillo’s arm slot naturally lends itself to attacking hitters more horizontally than his current arsenal allows and fits relatively well with the modern game’s addition of sweepers.

When examining pitchers who operate from similar arm angles (within three degrees) as Castillo’s, a common theme emerges: many complement their fastballs with a sweeper or another breaking ball that creates glove-side movement. Here are just six examples – three from either side of the rubber – to give an idea. You can go HERE if you’d like to dig in a little bit further into players with similar arm slots.

Castillo’s changeup and sinker can move arm-side. His four-seamer gives him a straighter, elevated look. However, his slider doesn’t create the sort of movement in the opposite direction that could force hitters to protect the entire horizontal width of the plate. A sweeper could change that by adding a different east/west option to his arsenal without having to completely remove his slider. In fact, it could potentially even make his slider a stronger pitch, as hitters would then have two different breaking ball shapes to consider.

As we’ve talked about, Castillo is a sort of enigma in many ways. His arm slot and arsenal generally reflect that of your typical east/west pitcher, but he relies pretty heavily on a vertical attack. By leaning a bit more into his arm slot and creating more of a horizontal differentiation in his arsenal, Castillo can “reinvent” himself without having to eliminate anything that’s brought him major league success for quite some time.

It feels like a pretty natural evolution for Castillo with where the game is today.

As Castillo gets ready to make his next start against Houston on Thursday afternoon, it is fair to be concerned that his best days are long behind him and that the team will be saddled with an expensive backend starter in what will hopefully be a competitive season. If you’re a fan of Stuff+, his score of 96 on the season is the lowest of his career, which tracks with what we’ve talked about already. While one game this afternoon won’t do much to shift the general opinion one way or another, what remains true is that Castillo’s track record is why finding another successful version of him is worth attempting. From 2019 through 2025, he established himself as one of baseball’s more dependable starting pitchers. But, the way he succeeds may need to change.

Pitchers rarely get to age without making adjustments, and nothing about Castillo suggests he will be one of the rare few for whom this occurs. For now, if the White Sox were to ask me what to do, I’d suggest not to reinvent Castillo yet, but rather refine what’s already there. Use his sinker to create a velocity options lower in the strike zone, make the changeup more difficult to identify, and find enough unpredictability to get Castillo through lineups multiple times.

Then, when the season ends, explore a larger transformation. See whether his release points need to be tightened. Determine whether his current slider remains the best breaking ball for his arm slot, or whether experimenting with a sweeper or another offering capable of providing more horizontal movement is the natural evolution of Castillo.

Assuming all goes well for the White Sox moving forward, they don’t really even need Castillo to become the pitcher he was five years ago. Ideally, this team would get other reinforcements, either internally or via free agency. But it will be up to Zach Bove and Brian Bannister to figure out what the 34-year-old version of Luis Castillo should look like.

And with a playoff race happening now and more than $22 million committed to him next season, they have plenty of incentive to find the answer.

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Featured Image: Chicago White Sox / X