How D-backs’ Justin Martínez came into his own

The D-backs found themselves one of the game’s elite relievers in 2024 with the breakout of Justin Martínez.

Propelling him to this breakout was his devastating sinker and a splitter that has only got better in 2025. At just 23 years old, the right-handed flamethrower is attempting to show last year wasn’t a fluke of any sort and continue his early season dominance this season.

Sinker

When Justin Martínez was originally called up to the D-backs in 2023, he struggled out of the gate. The Dominican pitcher limped to a 12.60 ERA across ten innings pitched. While he struck out more than a batter per inning in this span, he also walked more than a batter per inning too. What got him was a fastball that on the surface looked impressive at over 100 mph, but that ultimately had little bite to it, making it easy to hit.

Martínez went back to the lab to try and figure things out. What he settled on was developing a sinker which has played a dramatic role in unlocking the D-backs’ reliever’s potential. This year, he’s throwing the pitch even harder at an average of 100.4 miles per hour, right in line with his four-seamer which sits at 100.9. The four-seam fastball by itself was hittable, but combined with a pitch of similar speed in the sinker makes it harder for hitters to figure out in the guessing game that hitting can be.

The sinker’s effectiveness has been evident. The ground ball rate for Martínez jumped from 47 to 54 percent from 2023 to 2024, the ground ball-to-fly ball ratio went in a positive direction from 0.88 to 1.50 and in terms of Stuff+, the pitch ranked tenth best of all sinkers at 127. In 2025, nobody has been able to manage a hit off of it in the ten innings he’s pitched so far. While the pitch doesn’t generate a ton of whiffs, just 18.2 percent compared to his other pitches which sit in the mid-20’s to mid-30’s, it has been his most effective and go-to pitch.

Splitter

As the complement to his sinker, Martínez utilizes one of the most effective splitters in the majors and has become a go-to strikeout pitch out of the D-backs bullpen. It’s his sinker and fastball that catches eyes when you see the velocity reading at Chase Field when he comes out of the ‘pen, but the splitter is what really separates him from being simply a hard thrower to someone who can be consistently elite.

Typically what you want to see out of a pitch is a high spin rate which will induce more break to it, making it harder to hit. What Martínez’s does is the opposite, but still has the same effect. His splitter features knuckleball-like movement with an average of just 639RPMs, keeping hitters off balance. For comparison, it’s about half the spin rate of an average splitter, and his pitch with the most spin, the slider, averages 2,413RPMs. As MLB.com noticed, most splitters with as little spin as Martínez’s are generally thrown much slower, making his splitter a true outlier pitch.

Last year the spin rate wasn’t quite as low but still well below average like it is now and it was effective last year too. It was quite the potent pitch, boasting the highest put away percentage in two strike counts in 2024 at 36.5 percent. The splitter is mostly used against left-handed hitters to get outs whereas he uses his slider to get outs against the righties.

Control and contact

Last year, the main bugaboo for the D-backs reliever was the number of walks he surrendered, an 11.7 percent walk rate. This put him in the bottom 10 percent of pitchers. Early in the 2025 season, Martínez has been able to remedy this situation with a 3.4 percent walk rate. If this trend of locating pitches continues, Martínez may not be long for the co-closer role he’s shared with A.J. Puk (who is on the IL) so far this year.

Martínez has excelled at limiting hard contact. In his ten innings, he hasn’t allowed a single barrel or solidly hit ball. He’s allowing the lowest average exit velocity of his career at 76.4 miles per hour and he’s had the lowest sweet spot percentage of his career as well. Some may be concerned about the velocity being slightly down on his pitches in a recent outing or the extended rest he’s been given, Martinez says it’s nothing to worry about.

“The other day my velo was down a lot, but I don’t think it’s anything to worry about,” Martínez told Steve Gilbert of MLB.com. “My arm feels good, that’s the best part.”

The D-backs have seemed to find themselves a true gem in Martínez. With the rest of the surrounding core, the D-backs will be looking to improve upon their fourth place standing in the NL West.