Leahy's breakout gives Cardinals a rotation building block

When listing the best starting pitchers in baseball, you’re not likely to hear Kyle Leahy’s name thrown into the mix. But in his first full season in a Major League rotation, Leahy has grown into one of the most productive arms in the sport.

Among the 80 pitchers who’ve thrown at least 120 innings this season, Leahy’s 3.13 ERA ranks 16th, a testament to the development he’s seen on the mound. Things did not start well for Leahy, as he posted a 4.63 ERA through his first 14 starts and managed to complete the sixth inning in just one of those appearances. As questions surfaced about whether Leahy would be better suited to return to a bullpen role, the Cardinals never wavered in their confidence in him.

In 11 starts since, Leahy has a 1.28 ERA, the best in baseball in that stretch among pitchers with at least 50 innings. And now, even as Leahy is bumping against his innings restriction for the season, the 29-year-old has done plenty to show St. Louis that he should be firmly in its future rotation plans.

For the first half of the 2020s, the Cardinals relied on veteran rotations staffed with arms like Sonny Gray, Miles Mikolas, Erick Fedde, Lance Lynn, Kyle Gibson, Jack Flaherty, Jordan Montgomery, Jose Quintana and Adam Wainwright. That didn’t leave room for many of the young arms to get a look, including Leahy.

St. Louis revamped its player development and performance departments following the 2024 season and has started to see substantial growth from its young arms, leading the front office to open rotation spots for those pitchers to see what they can do in a Major League rotation.

Leahy’s background as a starter in the Minors and success in bulk innings out of the big league bullpen gave the Cardinals confidence that he could transition to the rotation, similar to what Matthew Liberatore did in 2025. Leahy, who had previously been a starter before moving to a relief role, entered Spring Training last year ready to compete for a starting role.

Leahy’s success on the mound is not just the result of batted-ball luck going his way. Since June, Leahy’s strikeout percentage has improved each month of the season and now sits at a 32% rate in August. His walk percentage has lowered at the same time, down to just 2.7% this month.

One clear reason for Leahy’s improved results has been adjustments in how he deploys his deep arsenal. Armed with six pitches, Leahy still threw his fastball more than 30% of the time in April and May, but he has since dropped that usage to closer to 20% by peppering in more of his secondary offerings. Leahy’s +7 breaking ball run value ranks in the 91st percentile this season, so it makes sense to lean on his strength while using the fastball and sinker to set up his sweeper, slider, curveball and changeup.

Leahy’s slider specifically has taken a major step forward this season. He has used it to put away hitters 29.2% of the time, the sixth-highest mark in MLB, while allowing just a .306 wOBA against it. It’s become Leahy’s bread-and-butter pitch, and he’s found ways to utilize it against both right-handed and left-handed hitters.

The Cardinals' rotation as a whole has outperformed expectations this season, but Leahy’s breakout campaign stands above the rest. Lately, he’s been showing the kind of strikeout stuff that could raise his ceiling as a starter, and he’s finding this level of success in just his first year on the mound. While that doesn’t guarantee that future seasons will bode better, it does create intrigue around how much more he can do in the future.

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Next season will give Leahy the chance to prove this season wasn’t a one-off, as well as push for an even higher innings output that could change how the league views him. It’s one thing to have one great season, but being able to stack successful campaigns and go deeper into his starts would be the next logical step in his development.

More exciting young arms are expected to join the Major League team in the coming seasons, but Leahy still has four years of club control remaining and might just be scratching the surface of what he’s capable of. The Cardinals player-development group has received heavy praise for its prospects and younger big leaguers' advancements over the past two years, but Leahy's late bloom might have a far more significant impact than many realize.

St. Louis shouldn’t expect Leahy to be a sub-2.00 ERA pitcher moving forward, but if the gains he’s seen on the mound during this recent stretch are sustainable, he could be someone the Cardinals can confidently rely on at or near the front of their rotation next season. And if Leahy continues his upward trend, he could quickly become a major part of St. Louis' future.

It’s safe to say Leahy’s first crack at being a Major League starting pitcher has gone as well as anyone could have hoped for.