Merrill Kelly Has Become the Diamondbacks’ Most Consistent Arm in 2025

"A great heartbeat, a great understanding of what it takes to go out and execute, nothing fazes him," said Diamondbacks manager Torey Lovullo. "Whether he went out and dialed up seven innings of one-hit baseball or he gets banged around for 4-5 runs, he's going to be the same guy walking in the dugout after the game, in the clubhouse, and a starter's work day tomorrow.

"There's consistency with his mentality, he's just super engaged and super prepared, and can flip the script and turn the page as good as anybody I know."

Kelly's consistency has kept the Diamondbacks' rotation afloat this year, as three of the other four starters have dealt with issues. Corbin Burnes has begun to round into form after battling shoulder issues for the first month of the season, Zac Gallen has been inconsistent at best, and Eduardo Rodríguez has been shelled in three of his last four starts before going on the injured list.

In his start against the Rockies on Sunday, the Diamondbacks needed Kelly to step up and set the tone. He did just that, with seven scoreless innings and struck out a season-high 11 hitters in a series-clinching win over Colorado.

Looking at his season as a whole, Kelly is putting up a solid 22.6% strikeout rate while walking just 6.5% of hitters. Both metrics are better than the major league averages of 22.0% and 7.3%. It's his highest strikeout rate since 2023. While the 16.1% difference between his strikeout and walk rate is slightly less than 2023, he could be on track to post the best mark of his career, depending on how successful he is the rest of the season.

Even though Kelly is averaging around 92 MPH on his fastball, his ability to command a deep arsenal of six pitches in and around the strike zone sets him apart from other starters who might have to depend on velocity to get swing-and-miss. On days he has a good feel for his changeup and cutter, he'll put up impressive strikeout totals. In his last two starts, he's punched out 19 hitters and allowed just one run in 14 innings.

"I think velo is deceiving, that's not always the only recipe for striking guys out," said Lovullo. "Merrill spins it, cuts it, sinks it, changes it, and does a really good job of setting up the hitter. You get them super engaged and then you got three or four weapons to continue just pouring it into the right spots at the right time."

"It's really good because he gives you a lot of options, a lot of weapons to play with, and I always try to take advantage of that," Diamondbacks catcher José Herrera told Arizona Diamondbacks on SI's Jack Sommers. "He's a great pitcher, he commands the zone very well, that makes my life a lot easier."

The changeup has been the best weapon in Kelly's arsenal since he introduced a revamped version of it in 2023. In 2025, opposing hitters are batting .183 with a 33.3% whiff rate against his changeup. It only has about a velocity difference of 4 MPH compared to his three different fastballs, but has an above-average 35.2 inches of drop, which allows the pitch to sink below the swing path of opposing hitters.

"That's the pitch I go to when I need something, it always has been," said Kelly. "The action on it has been really good as of late. When it's on, it's one of the pitches I go to when I need one. If I can locate it, it's a super tough pitch to hit for a couple different reasons. It looks like the fastball, similar speed as the fastball, and I try to make it look like the fastball as much as I can."

The right-hander has flown a bit under the radar over the years, despite being an instrumental piece in the Diamondbacks' run to the World Series in 2023. That postseason, he averaged six innings a start and pitched to a 2.25 ERA, as Arizona won three of his four starts. However, this could be the season where his consistency and success lead to more recognition across the league.

When looking at the Diamondbacks' roster, Corbin Carroll and Geraldo Perdomo seem like obvious candidates to go to the All-Star Game in Atlanta. Kelly has fared well compared to his peers, as he ranks seventh among National League starters with 1.7 bWAR. He ranks in the Top 10 in WHIP, Hits Per Nine, and Innings Pitched. Another strong month could result in his first career All-Star selection.

Receiving All-Star recognition would be another great achievement to add to an already impressive perseverance story. Kelly took an unconventional route to the big leagues, including a stop with the SK Wyverns in the Korean Baseball Organization for four years before getting signed by the Diamondbacks. His success has already blazed a trail for others to follow, proving that playing overseas can be a successful development path for players with major league aspirations.