It's time to end the Mitch Keller cutter experiment

Pittsburgh Pirates’ starting pitcher Mitch Keller started throwing a cutter last season. However, after using it as a secondary offering in the first half of the 2023 season, it became an extremely prominent part of his arsenal. But this auspiciously happened right around the time Keller started having inconsistent outings. With that being said, it’s time to end the Mitch Keller cutter experiment and put the pitch back on the back burner.

Through Keller’s first 22 starts of 2023, he had a solid 3.97 ERA, 3.79 FIP, and 1.21 WHIP. He was striking out about a quarter of opponents faced with a 25.7% K% with a strong 7.1% walk rate. He also had an above-average 1.14 HR/9 and 86.9 MPH exit velocity. His cutter made up about 22.3% of the pitches he threw in this span. His four-seam fastball and sinker were both very prominent parts of his arsenal, with both his slider and curveball also being used at least 10% of the time.

But after this point, Keller saw his numbers move in the wrong direction. He still had a solid 3.82 FIP, 24.9% K%, 5.7% BB%, and 1.19 HR/9, however, batters were hitting the ball harder with an 89.4 MPH exit velocity and were allowing more earned runs with a 4.75 ERA. A .339 batting average on balls in play wasn’t doing him any favors, but part of that is because he was also giving up more line drives. During this stretch, the cutter became a very prominent part of his pitch usage, as he threw it 28.7% of the time. He was still using his four-seamer and sinker a lot, but his curveball was rarely used as it’s usage dropped below 5%.

Overall, Keller’s cutter was hit for a .318 batting average, .506 slugging percentage, and .374 wOBA. The only things that were somewhat positive are that his xwOBA was significantly lower than his wOBA at .354, which still is below average, and he induced a lot of soft contact with the offering. Batters only managed an 86.1 MPH exit velocity against the pitch with a 30.7% hard-hit rate.

Now it’s not like Keller’s cutter was always bad. Through the first three months of the 2023 season, Keller’s cutter held opponents to a wOBA well below .300. However, through the final three months of the season, when the cutter became a primary part of his pitch selection, batters hit the offering for a wOBA over .450. There is such a thing as too much of a good thing. Part of the reason Keller’s cutter may have been as effective as it was is because of its ability to play off his other pitches.

Keller struggled in his first game of the season. He allowed four earned runs, walked two, and only struck out three in 5.2 innings against the Miami Marlins in the season opener. The cutter was used more often than any other pitch in his arsenal as he used it over 30% of the time (31.8% to be exact). His cutter induced eight batted balls with an exit velocity averaging 92.9 MPH.

During Wednesday’s game, the Pirates opening day starter Keller allowed four earned runs in the second inning.

In that inning, he threw seven cutters, two of which led to hits that scored runs (Joey Gallo solo homer and CJ Abrams RBI single). However, he would pitch four and a third innings where he would allow only one earned run with five strikeouts while only throwing 11 cutters. Of course, 4.1 innings isn’t perfect evidence, but considering everything else, I don’t think it’s all that much of a coincidence Keller was more locked in when he was throwing significantly fewer cutters.

I’m not the only one who is pointing this out, both Alex Stumpf of MLB.com and our very own site expert Ethan Fisher have both pointed out on Twitter/X that Keller’s cutter has been far from his strong suit. To Keller’s credit, maybe he’s done playing around with the cutter. In his final three and a third innings of Wednesday’s game, he threw 43 pitches with just four being cut fastballs, which equates to 9.3% of his pitches. Hopefully, this has been a learning experience for Keller and he can go back to making his cutter more of a secondary offering than a primary offering.