It has been just over one year since Mitch Keller returned to the Pirates' rotation on the last day of May in 2022, and that story has been well-documented by this point. He emerged from his demotion to the bullpen with a new sinker and wasn't afraid to use it, which was the first of several changes he would make over the next year (introducing a sweeper, revamping his cutter) that has helped him live up to his former top prospect billing.
Through the first two months of the season, Keller is 7-1 with a 3.25 ERA and 93 strikeouts over 74 2/3 innings. He's on pace for a 20-win season and is making a strong case for his first All-Star nod. If those last four months of 2022 showed what Keller can do, this year has been him delivering on his potential.
Like last year, the sinker is a big part of his success. Not just because he's getting strikeouts with it, but because he is using it in a new way and it has helped him finally handle those pesky left-handed hitters.
For years, lefties had the upper hand against Keller. As a rookie in 2019, southpaw swingers had a .431 on-base percentage against him. In 2021, it was .419. Even in that big leap forward last year, he allowed a .359 on-base percentage to lefties, and it was actually a little worse (.365) from May 31 to the end of the season.
This year, left-handers are getting on base at just a .295 clip against him. A big reason for that is Keller isn't giving them the free trips to first base anymore. Two of the three "true outcomes," or what a pitcher can control, for a pitcher are walks and strikeouts. Look at how many strikeouts Keller is averaging per walk against southpaws in 2023 compared to the last four seasons:
2020: 0.55 SO/W
2021: 1.22 SO/W
2022: 1.62 SO/W
2023: 5.4 SO/W
Keller walked 34 lefties last year with 55 strikeouts. He's already at 54 southpaw strikeouts this year, but with just 10 walks. Oscar Marin and the pitching team have greatly emphasized strike percentage and minimizing walks, and Keller is a poster boy for what can happen when you just throw strikes.
The sinker plays into that, even if Keller has thrown only 49 sinkers to left-handers al year. Of those 49 pitches, though, 18 have been a strike three. The sinker is usually a pitch you go to when you need contact, like a double play. For Keller, it's a kill pitch.
So how is it a strikeout pitch? It's because 14 of those 18 strikeouts have been looking.
Keller is getting called strikeouts at a ridiculous pace this season. He leads the league with 40 strikeouts looking, well ahead of the second-place Gerrit Cole with 29. He got 23 punchouts this way in May, which is the fourth-most in a month for any pitcher in the pitch tracking era (since 2008):
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The sinker may not be the go-to pitch for those called third strikes, but it is the most efficient. A big reason why is how Keller is using it, especially against lefties. Keller has taken inspiration from how JT Brubaker would front hip his two-seamer, or ride it inside to his glove side. Keller can emulate that because he has a cutter which plays off of it, which helps freeze hitters.
"We call it 'X-ing it' with the cutter," Jason Delay would tell me. "Cutter in on the hands with the two-seam -- I like it a little more at the belt -- it makes it really tough for lefties to look for one pitch."
To visualize this, take a look at Keller's at-bat against the Orioles' Anthony Santander last month. It's a three-pitch sequence, so ignore that pitch on Keller's hand side and focus on the two running in on the lefty. Those purple dots are the decision point for Santander, where he has to choose if he wants to swing or not. Looking at the cutter (the red tail, or the one starting in the middle) and sinker (orange), you can see that point where they cross after that decision point, creating the X:
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For full disclosure, Santander was able to take that sinker for an 0-2 base hit. This is more of a visual example, plus proof that Keller is willing to go to a pitch that's usually used for quick contact when ahead 0-2 because it's a good strikeout pitch. He has that much confidence in this sequence.
"Make them decide," Keller explained. "It's part of the design."
This is just one many tools Keller has at his disposal with his arsenal, but it's a dangerous one. Not to mention, it's one that has built on the foundation he built in the second-half of last year.
"The maturation of Mitch Keller has been one of the most exciting things that we’ve had this year," Derek Shelton said. "The way he goes out and competes is really important. I think we’ve really seen him grow up and do that."
