The latest and best iteration of Casey Mize (And his journey is not complete)
During Casey Mize’s first two career postseason starts this October, the benefits of his continued development were clear.
On Oct. 1, the Detroit Tigers’ right-hander allowed just one hit and one run over three innings in the second game of their wild-card series at Cleveland.
In the second inning on a blustery afternoon in Cleveland, Bo Naylor swung and missed over his new slider to strike out. Another lefty, rookie Chase DeLauter, whiffed on a tumbling splitter and again on a 95 mph four-seamer. Fifty-eight of his 62 pitches were against left-handed batters.
He could have gone deeper into the game as he had averaged 5.3 innings per outing over his 28 regular season starts, but it was a game Tigers manager A.J. Hinch managed like an elimination contest, considering the best-of-five format.
On Oct. 9, he made his second career playoff start versus Seattle.
While it was another brief appearance, it was even more impressive as he struck out six Mariners over three innings of one-run ball. He induced five whiffs on his splitter, four on his two-seamer. Many, at least on social media, were again surprised that Hinch pulled him early, after just 54 pitches. This outing came against a more balanced lineup: 24 pitches versus righties, 30 versus lefties.
A season earlier, Mize was not in a position to start a playoff game, or combat a variety of lineups, let alone post career bests in fWAR (2.4), strikeout rate (22%), ERA (3.87) and expected ERA (3.70) as he did in the regular season. He even made his first All-Star Game.
Mize’s 2025 was one of progress. But it’s also just one chapter, one considerable step. His developmental journey is not complete.
Mize was not happy with his 2024 campaign.
He was not satisfied with his 4.49 ERA, or his 16% strikeout rate – each worse than the league average.
Always open to information, the former No. 1 overall pick out of Auburn set out after the season to look for answers and solutions.
“I went to different places just to see what the plan should be, kind of bouncing ideas around,” Mize said. “Getting different ideas on what I could do going into the offseason trying to prepare for the season.”
That curiosity brought Mize to Driveline.
Just five days after last season ended, he visited Driveline’s Arizona facility for a motion capture of his delivery. The plan to reconvene later in the winter after his break from throwing.
What Mize had in his favor was above-average command, above-average extension, and slightly above-average velocity for a starting pitcher. Those are all valuable traits from which to build from.
But he knew he had to expand his arsenal. He was not sure of the best way to do it.
“If you have two plus pitches, good for you,” Mize said. “I don’t necessarily do, so I feel like I have to have a wider range of possibilities to combat a lineup.”
As Mize took a break from throwing, Driveline pitching coordinator Matthew Kress investigated his data and profile. Mize’s mechanics and strength assessments were in good shape but a couple elements jumped out to Kress.
“It was just very apparent going through his data that one of the bigger issues was (lacking) any sort of breaking ball that would turn left,” Kress said.
Mize had a put-away rate of 12.6% against right-handed batters, well below the league average.
“The issue is putting them away and finishing them, which screamed ‘Hey, you need some sort of breaking ball to attack righties,” Kress said.
The assessment results were not a total surprise to Mize.
“I’ve never been able to make the ball go left, especially with my slider,” said Mize, noting he’s often had reverse splits.
Mize throws with excellent spin efficiency, working behind the baseball well. But Kress explains that it can make it difficult to create the big east-west shapes we’ve seen proliferate through the game in recent years.
“It’s probably a little bit tougher than for somebody who has more of a lack of spin efficiency,” Kress explained. “It doesn’t mean that it’s impossible. It just means that we have to do a lot of things right.”
By December, Mize was ready to throw and visited Driveline’s Tampa facility located near his offseason home in Florida.
Mize is obsessed with details.
For instance, when he first walked into our new facility in Tampa, Mize noted to Kress that he’d like a tighter zoom from the Edgertronic camera.
“The next thing you know Kyle (Boddy) had purchased an Edger lens to get us an even crisper view of what was going on,” Kress said. “Kyle likes to joke that (Mize) forced us to make our Edgers even better down in Florida. He’s not wrong.”
Mize entered the offseason with a tighter, smaller slider in his arsenal. But he had to be precise with it: start it at the edge of the plate and hope for a whiff or weak contact. He needed something bigger. Ideally, a sweeper.
Mize and Kress experimented with different grips under the watch of the Edgertronic, including the two most common sweeper grips: the “Whirly,” which the Yankees popularized in 2021, and what we label the “SL 5” at Driveline, which is a designation of how the grip is positioned on the horseshoe seams of the baseball.
After offseason work with the grips, Mize entered the season with a slider that broke more than his existing breaking ball, but one that didn’t break as much as a true sweeper.
There was marked progress with the pitch but not perfection.
In April, the pitch averaged 4.9 inches of glove-side movement, according to Baseball Savant data. In May, it improved to 6.5 inches, and 7.9 in June where it plateaued. It averaged just under 7.5 inches for the second half of the season.
It remained a work-in-progress during the season.
“The issue — and I could see it in a lot of slow-motion video that either he would provide, or that the broadcast would provide — was that he was spinning on what I call a white patch (of the baseball),” Kress said. “When it comes to trying to produce the (sweeper) movement quality we want, we really need to lean into something called a one-seam orientation. So, what I had said to him was for us to get the maximum amount of sweep, I think that we’re going to have to lean heavily into this one-seam orientation.”
To help Mize harness the correct seam orientation Kress shipped him a product called Seam Stamps created by Mariners pitching coordinator Quinn Cleary. The tool is a baseball-shaped stencil that gives pitchers immediate external feedback about the seam orientation of each throw they make.
“It does help (pitchers) better understand how their ball is moving through the air,” Kress said. “‘These are your circles you’re making (with the stencil). This is where they should be. This is what we should see.’ Those sorts of things.”
But even as the average movement leveled out, his expected wOBA with the bigger slider improved as the year went along, posting his best wOBAs with the pitch in August (.274) and September (.247).
Progress. Building something new in a lab still requires learning to integrate it into games.
“I had a ton of glove-side misses early on,” Mize said of his early season. “We had to work through some of the intended target (locations). I was used to having to work on the black, away with the slider. Now, I had to trust myself to throw the ball more over the plate, which was terrifying.
“I was not used to thinking ‘Hey, throw your slider over the plate and it will go where you want.’ It took a minute for me to trust that. I had a lot of (miss) from not believing in that yet. So, I think that will be another layer. Perhaps Intended Zones can help harness that.”
His command numbers were especially strong later in the season.
Overall, the pitch helped him increase his put-away rate against right-handed batters from 12.6% in 2024 to 17.2% this season. His put-away rate against lefties increased from 18.7% to 21.4%.
The pitch was a success in that it added new shape in his arsenal, an offering he threw 14% of the time this season. It created measurable improvement in his overall performance. But Kress and Mize still want more from it. It is and remains a work in progress.
Said Kress: “We’re still working on it and even with the second offseason, hopefully, will be able to spend some more time (perfecting) it.”
While the sweeper was a focus, so was refining the existing parts of his pitch mix.
Mize knew his four-seam fastball had lost carry, which was something he wanted to try and address. He also knew it was the most difficult pitch to alter.
“We literally just flipped the seams on the ball to try to get more induced vertical break,” Kress said. “This is effectively another seam-orientation trick, trying to produce more carry. It was seen last season by Luke Weaver, where the Yankees changed the seam orientation on Luke Weaver’s fastball and made an enormous jump. That was because he had a very weird fastball grip that was out of the ordinary.”
Mize’s fastball gained an inch of vertical movement this season, even with a slight velocity drop. It was not a major change, but more incremental progress.
Mize was also curious about what Kress would say about his splitter.
His signature swing-and-miss pitch coming out of Auburn had not performed as an elite offering in professional baseball.
“I went in there ‘OK, what are going to do about my split?’” Mize recalled asking.
Initially, the recommendation was to throw the pitch with more velocity. But after uneven results, about halfway through the season, Kress suggested another change: and that was to add more cut, more gyroscopic spin to the pitch.
What Kress has learned about the changeup is differentiating its vertical movement from a pitcher’s primary offering is key.
“When you kill the vertical break, it gets further away from the four-seam cloud, or the four-seam movement profile, and that’s the driver of more swings and misses,” Kress said. “In his case, when he’s able to run it down towards the zero line, the zero vertical line, it just plays so well.”
On the surface, his splitter took a step back this year.
The run values on his splitter (-1.96 wFS per 100 pitches) suggest he should be throwing it less often, not more frequently as he has done in 2025, especially later in the season.
But Mize explains why evaluating standalone pitch values can be problematic.
“I give up a homer on the pitch, and that’s going to negatively affect the value, any type of slug, which is what happens sometimes if I leave (the splitter) up in the zone,” Mize said. “But I threw 28 (splitters) in a particular game and I get eight swings and misses on it. The run value went down because of the damage that happened on a home run and double. I understand how they calculate that, but if you look at it and say ‘Oh, your run value is bad you need to scrap that pitch,’ like, I would be worse without that pitch. I need to throw that pitch.”
For instance, while his splitter’s run value decreased – his fastball finished with a slight positive run value for the first time his rookie year in 2020. This speaks to the power of a wider menu of pitch offerings.
“I think the bigger thing might be the fact that we added more pitches … allowing us to not have to rely on throwing as many four-seamers and therefore letting it produce a little bit better,” Kress said.
Mize finished the season with a five-pitch arsenal, throwing no pitch more often than 34% of the time, and his four-seamer, but throwing each pitch at least 12% of the time. He threw only three pitches at a double-digit clip in 2024.
Even though most of Mize’s offerings do not grade out as wipeout offerings – the combination of pitches in an expanded pitch mix raised his entire game.
And 2025 is just one chapter. Mize and Kress are curious to see where his pitch mix is with another offseason of improvement.
Mize’s story is one of iterating.
It’s also one that demonstrates how coaching staffs both inside and outside of affiliated baseball can work together in harmony.
For instance, Mize believes the Tigers enjoy excellent pitching coaches. They also have access to Hawkeye data that those outside affiliated baseball do not enjoy.
“They do a really good job of reviewing Hawkeye data and seeing how I am moving, trying to get ahead of some mechanical stuff that can bleed over from time to time,” Mize said. “We’ve been able to correct some things before they become too bad of trends which has been super beneficial this year. ‘Hey, you are starting to counter rotate too much,’ or, ‘You are flying open.’”
But he also wanted to go outside to hear other opinions on how to optimize his arsenal and where he could improve. The Tigers signed off on his plan.
“Combining all those things is what has allowed me to take a step forward this year,” Mize said.
And this year is just one chapter. Mize’s development story is not complete. It’s a journey that will continue this offseason, with another breakthrough, another performance jump, well within the range of possibility.
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