Diamondbacks Merrill Kelly Learning to Trust the Slider

The art of pitching can be a difficult thing for a layman to understand. Even those of us who write about and study the game closely can miss key elements to what makes a pitcher effective.

When it comes to a pitcher like Arizona Diamondbacks starter Merrill Kelly, mastering the craft has been a years-long process, honing his talents together with his intelligence and work ethic to improve.

Kelly has never been a hard thrower. While he can top out around 95 with his four-seamer and sinker, he typically averages around 92. Those pitches have never been high swing-and-miss offerings, he gets those with his renowned changeup.

Kelly also throws a cutter and curveball, and since 2023, has began introducing a slider into the mix. Utilizing this six-pitch mix, and possessing excellent command, Kelly has carved a very successful major league career since returning from Korea in 2019.

Oftentimes we look at pitch metrics and focus on the results on those pitches, or metrics such as velocity, spin, break, etc. We look at the stuff and results, and try to determine if it's a good pitch or not. But for a true artist on the mound like Kelly, it's about more than that. How the pitches play off each other, and how one pitch sets up the other is a key element to success.

For Kelly, the slider is just such a pitch that can enhance his repertoire in ways beyond the results he gets on the pitch. Introduced in 2023, Kelly used it 6% of the time and increased that to 12% in 2024. The WHIFF rates were impressive at about 40% each year. In Thursday's game against the Seattle Mariners he threw eight of them, generating four WHIFFS on five swings.

Pitching Coach Brian Kaplan has been trying to get Kelly to use the slider more, to the point of harping on it on the bench during Thursday's game. He's dong so for a very specific reason beyond the expected results on the pitch. It's because Kaplan believes the slider will help make his sinker more effective.

"[Kaplan] thinks it can protect the sinker," Kelly said. "He thinks if I'm going to miss with the sinker, it gives me maybe a split second of questions that [hitters] have to wait. They have to wait and see if it's going to sink. And that might give me just that millisecond of pause that I might need to get them off the barrel if I do happen to miss with it."

"So I think he thinks it's a sinker protector is what he called it. That if I do miss with it, it gives me more of a window to miss because they have to now make sure that it's not going to go the other way."

That's a lot to unpack, but to simplify it, the sinker tails in and down towards a right-hand batter, while the slider moves away. To gain a better visual of this effect, you can go to Kelly's Baseball Savant Page and view the Movement Profile graphic at the top right of the page.

Kelly is 36 years old and coming off a season marred by a shoulder injury. But he came to camp healed and his stuff looks better than ever so far in spring. He was one of the most effective pitchers in the game in 2022 and 2023. It's a good bet he will be so again in 2025, providing the Arizona Diamondbacks with three potential aces at the top of the rotation along with Corbin Burnes and Zac Gallen.

A free agent at the end of the year, Kelly would like to finish his career in Arizona if at all possible. A product of Desert Mountain High School in Scottsdale, and Arizona State University, he lives in Arizona year round. It would be hard to imagine him in a different uniform. But those are concerns for another day. For now, Diamondbacks fans should enjoy getting to watch a master at his craft weave his magic for as long as they can.

Diamondbacks Merrill Kelly Learning to Trust the Slider

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