When 21-year-old Justin Martinez toes the rubber in the majors, he will become the youngest Diamondbacks pitcher to debut since Tyler Skaggs on Aug. 22, 2012. He could also become the hardest throwing pitcher in D-backs history, sporting a four-seam fastball that has averaged 99.6 mph — and topped out at 102.5 mph — in the minors this year.

The Diamondbacks recalled Martinez on Tuesday along with right-hander Kevin Ginkel. In a corresponding move, they optioned lefty Joe Mantiply to Reno and placed starting pitcher Merrill Kelly on the 15-day injured list with a blood clot in his lower right leg.

Martinez stormed through three minor league levels in 2023 and finished the year with a strong showing in the Arizona Fall League. The D-backs awarded him a 40-man roster spot in the offseason, and he was a candidate to make the Opening Day roster out of spring training. He was ultimately optioned out of major league camp on March 14.

At first glance, Martinez’s promotion appears to be oddly timed. His numbers in Triple-A Reno this year do not jump off the page. In 28 innings, he has a 4.18 ERA and 1.71 WHIP with 27 walks compared to 38 strikeouts. Martinez has walked nearly a batter an inning this season.

However, his control — and overall results — have improved significantly of late. Since April 17, Martinez is 2-0 with a 0.39 ERA, 1.21 WHIP, a .167 opponent batting average and 30 strikeouts compared to 15 walks in 23 innings.

Hold on — how does a guy with a 4.18 ERA for the season have a 0.39 ERA over that long a span? Simple: He gave up 12 runs (all earned) on eight hits, 12 walks and a hit batter in his first five innings pitched of the season.

Granted, Martinez’s 15 walks over 23 innings since April 17 are still less than desirable. That works out to a 16.1 percent walk rate, which is nearly twice the current major league average.

When asked what will be most crucial to his success in 2023, Martinez was quick to point to limiting walks and attacking the strike zone as a necessity. D-backs manager Torey Lovullo reiterated the point.

“I know it’s 100, 101, 102, and I love that,” Lovullo said. “I love that power. But it’s about throwing it where you want to and what are you going to do off of that, that’s the most important thing. And that’s what he’s been doing very well lately in Reno.”

Frankly, it is all but certain that Martinez will struggle with command to some extent in the majors. He has carried far above-average walk rates throughout his minor league career.

That said, Martinez’s stuff is good enough to give him a fighting chance of turning into a high-leverage bullpen arm in spite of his control issues. Along with a four-seamer that routinely sits in the triple digits, Martinez also throws a splitter and slider, both of which sit in the upper 80s.

Ironically, Martinez’s four-seamer does not really function as a strikeout pitch. According to Brooks Baseball, Martinez’s four-seam fastball has generated whiffs on 17.9 percent of swings.

For reference, the whiff rate on Scott McGough’s four-seamer — which is averaging 93.8 mph this year — is a nearly identical 18 percent. Suffice it to say that velocity does not necessarily equate to swing-and-miss, and Martinez is a clear example of that.

Fastball shape is really important, too, and that’s where Martinez’s heater is less ideal. In 2023, his four-seamer is averaging 6.8 inches of vertical ride. For reference, McGough’s four-seamer is averaging 9.8 inches of vertical ride in 2023. A four-seamer with below-average vertical ride will have less effectiveness in the upper part of the strike zone, where fastball whiffs often come.

Of course, that doesn’t mean Martinez’s blazing velocity won’t generate a whiff every now and then — and make opposing hitters look silly in the process.

It is worth noting that, while Martinez’s four-seamer is not likely to be an elite swing-and-miss pitch, 69.8 percent of batted balls against it have been on the ground. Somehow, opposing hitters are still hitting .327 with .473 slugging percentage against it. Of course, this is the Pacific Coast League we are talking about, so those numbers should be taken with a grain of salt.

In spite of Martinez’s flashy fastball radar gun readings, his secondaries are really what sets him apart as a potential high-leverage arm. The splitter is his bread-and-butter.

Martinez’s splitter has generated an impressive 46.5 percent whiff rate in 2023. For reference, McGough’s splitter — which has been one of the best in baseball to date — has a 40.4 percent whiff rate.

Although public scouting reports strongly favor Martinez’s splitter over his slider, he is using his splitter only 7.4 percent of the time against right-handed hitters. It is, however, his primary out-pitch to lefties with a usage rate of 32.8 percent.

In addition to the aforementioned whiff rate, opposing hitters are batting only .042 against Martinez’s splitter with a .125 slugging percentage.

Martinez first started using the pitch last year upon returning from Tommy John surgery in 2022. In a newly released batch of prospect rankings, Eric Longenhagen of FanGraphs gave Martinez’s splitter a future grade of 70 on the 20-80 scouting scale. It has a chance to be one of the best off-speed pitches in baseball.

That leads us to Martinez’s slider, which he is throwing 38.1 percent of the time to righties and 11.6 percent of the time to lefties. Public evaluators are not particularly high on it, but it has a ridiculous 66.1 percent whiff rate in 2023. On top of that, opposing hitters are hitting just .095 against it with an .095 slugging percentage.

It is too early to draw any definitive conclusions, but it is at least worth considering the possibility that Martinez’s slider is more than the fringy third pitch it has been perceived to be. Martinez has relied on the pitch heavily against righties this year, often driving it out of the zone down and away.

Ultimately, Martinez is something of a wild card for the D-backs. His command looks like a significant obstacle, and it may keep him from reaching the two-strike counts he needs to get the most out of his secondaries.

Still, Martinez’s raw stuff is as good as any reliever in the D-backs’ organization, and it is not out of the question that he could be closing games for the D-backs in the not-too-distant future.

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Top photo: Mark J. Rebilas/USA TODAY Sports