Will Merrill Kelly Return to Being the Mainstay of the Rotation?

How is the veteran starting pitcher projected to do in his last year under contract with the Diamondbacks?
Sep 16, 2024; Denver, Colorado, USA;  Arizona Diamondbacks starting pitcher Merrill Kelly (29) delivers a pitch in the first inning against the Colorado Rockies at Coors Field. Mandatory Credit: Ron Chenoy-Imagn Images
Sep 16, 2024; Denver, Colorado, USA; Arizona Diamondbacks starting pitcher Merrill Kelly (29) delivers a pitch in the first inning against the Colorado Rockies at Coors Field. Mandatory Credit: Ron Chenoy-Imagn Images / Ron Chenoy-Imagn Images
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Continuing our series of Arizona Diamondbacks player reviews, steady starting pitcher Merrill Kelly is up next. The projection below is a composite of ZiPS and Steamer projections, prorated to our playing time projection.

Merrill Kelly, Right-Handed Starting Pitcher, Age - 36

Merrill Kelly has been one of the most consistent starting pitchers in Diamondbacks history, especially over the last few years, although 2024 did have a wrench thrown his way. His shoulder injury cost him plenty of time and forced him to overcome some rust once he returned.

However, he finished the year strong over the last month and set himself up for a potential big 2025 season. Plus, he's going to be playing motivated as this is the last year of his contract with the Diamondbacks.

While he's spoken publicly about seeking an extension with Arizona, he likely has to prove he's healthy and performing to get that extension or to get a big money deal on the free agent market ahead of his age-37 season.

It will be his last shot to get a major dollar deal likely, but first he has to perform as he has throughout his career.

Merrill Kelly career stats with the Arizona Diamondbacks and his 2025 projection with the D-backs.
Merrill Kelly career stats with the Arizona Diamondbacks and his 2025 projection with the D-backs. / Jake Oliver

Why Kelly may outperform the projection

Kelly has outperformed the metrics above in three of the last five seasons. Had he not gotten hurt last year, he likely would've performed far better since it's unlikely he would have had a disastrous August.

He's pitched more than 127 innings in all of his seasons that weren't the covid-shortened season or last year.

That's four seasons in which the lowest he pitched was 158 innings. As long as Kelly is healthy, he absolutely should be able to pitch more than what is projected and start far more than 24 games.

After all, aside from 2020 and last season, the fewest games he started in a season were 27. He earned the nickname "Merrill the Mainstay" for a reason, and that's due to him being able to pitch deeper into games every week and be available for nearly the whole season.

Kelly's FIP has been 4.25 or lower in each of the last five seasons and in four of those five, it's been lower than the 4.14 projection.

Kelly's ERA has been 4.03 or lower in all but one of his seasons since 2020. He knows how to limit runs and prevent hitters from getting on base. A 4.04 would be very close to last year, but if he's healthy and performing, he should finish in the 3.00's.

Kelly has averaged 2.625 aWAR every year of his career and aside from last year, he's put up better than 2 WAR each of the seasons from 2021-23. 1.6 does seem plenty reachable so long as Kelly is healthy.

Had he not gotten hurt last year, Kelly would've made around 30 starts. If he had, he would've been projected for 1.96 aWAR based off how much WAR he had accumulated over the 13 starts he did make.

Kelly has struck out at least 21% of the batters he's faced in every season since 2021 and he's done it in four of the six seasons he's played. While his strikeout rate went down from 25.9% to 21% between 2023 and 2024, that isn't too alarming.

He was recovering from a shoulder injury, still getting control of his pitches, and he's made different leaps and drop-offs in his career before.

In 2022, his strikeout rate was 22% which saw a steady increase from 2021 when it was 19.%. However, in 2020 it was 23.2% so it fell off from year to year.

His walk rate last year 6.3% and it's been lower than 7.7% in all but one of his seasons. The walk rate listed above is very reasonable but if he performs similar as last year, he should come well under the projection.

Why Kelly might underperform the projection

The two biggest factors going against Kelly, that could cause him to fall short of these projections, are age and health. He's entering his age-36 season. It's quite common for non-Hall of Fame pitchers to start seeing their pitches and stuff decline around this age.

Of course, Kelly was a late bloomer and there are always exceptions to the rule such as Alex Cobb or Charlie Morton.

Plus, Kelly takes care of himself and is one of the hardest working players on the team. However, Father Time is undefeated and this could be the season that sees him slip the tiniest bit.

In which case, he could finish around these numbers or finish slightly worse than them. He'd still be a quality starting pitcher, but not the co-ace he was the last couple of seasons.

The other factor is injury. From the blood clot in 2023 to cramps to the shoulder injury, he has endured a lot over the last two seasons. He's healthy now, but injuries can pop up out of nowhere.

They happen when it's least expected so while one can't accurately project for an injury, it is a very real concern especially for older pitchers.

Luckily for Kelly, he relies on control and movement versus high-velocity pitches so he is not as high a risk for an elbow injury as say Justin Verlander who threw upper-90s in his late-30's.

Still, any missed time aside from a couple of weeks could see Kelly fall short of reaching the WAR, games, and innings threshold.

The vast majority of Kelly's advanced stats were close enough or similar to years past that there isn't a clear indicator that he will struggle mightily this season.

Summary

Kelly is a veteran starting pitcher with years of high-level performance and the knowledge of how to get batters out in a multitude of ways. He has six pitches and the ability to land any of them for a strike when he needs one.

Kelly has proven to be a pitcher capable of being one of the best No.2s or 3s in MLB over the last few years. Aside from a terrible month, his stats last season would've been far better.

However, he is entering his age-36 season and the injuries can quickly happen and stack up fast.

Plus, as he gets older, he will slowly lose his ability as does every pitcher. However, he certainly could be the next Charlie Morton who is a strong pitcher into his 40's, especially since Kelly doesn't rely on velocity.

Only time will tell, but for now, there's far more statistics that say he will outperform his projections for the 2025 season with the Arizona Diamondbacks than there are that say he will fall short.

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Arizona Diamondbacks 2025 Player Projections Links Hub


Published
Jake Oliver
JAKE OLIVER

Jake Oliver is a Baseball Reporter for Arizona Diamondbacks On SI. He is the site's prospects writer and an editor. He is the former site expert of Venom Strikes and has been featured on numerous websites and podcasts. Jake has been a reporter for four years. He holds a degree from Paradise Valley Community College and lives in Arizona. Follow him on X for breaking news and more coverage @DarthDbacks