Joe Mantiply has quietly become one of the most reliable left-handed relievers in baseball

If you said in March that the Diamondbacks’ best reliever after two months was going to be Joe Mantiply, I would have been unhappy with the direction of the team. Here we are after 51 games in which the team has played surprisingly competent baseball to the record of 25-26, in large part due to Mantiply’s contributions. The 31-year-old journeyman reliever seems to have found a long term home with the Arizona Diamondbacks thanks to a near full year of surprisingly strong pitching and has entrenched himself as one of the most trusted arms in the bullpen in 2022.

Previously a 27th round pick out of Virginia Tech, Mantiply quickly moved to the bullpen early in his Minor League career before bouncing around three different organizations. The D-backs signed him to a Minor League deal after the 2019 season with an invite to Spring Training. He got a chance to pitch out of the bullpen in 2020, but would give up 4 runs in 2 ¹⁄₃ innings before getting outrighted off the 40-man roster. After failing to make the Opening Day roster last year, Mantiply made 5 appearances for Reno before getting promoted to the MLB roster for good.

In his first 14 appearances, Mantiply put up decent numbers with a 2.92 ERA but it had pretty concerning peripherals. Despite the run prevention, Mantiply had a WHIP of 1.62 thanks to a 12.7% walk rate and a .260 opponent batting average. Those numbers were not sustainable in the least without an improvement in at least one of those two areas. This problem came to a head in a game against the Los Angeles Dodgers on June 18th in which he failed to retire any of the three batters he faced and all three of them scoring. That pushed his ERA to 5.11 on the season during the middle of the team’s tailspin in May-June 2021.

Fortunately for the D-backs, Mantiply bounced back from that outing in a big way. From June 20th, his next appearance, to the rest of the season Mantiply pitched to a 2.63 ERA. In addition to improved run prevention, the underlying metrics also significantly improved. His strikeout rate improved from 20.2% to 23.0% while cutting his walk rate in half from 12.7% to 5.3%. At the same time, his ground ball rate spiked to 51.9% in that time period and his hard hit rate dropped to an impressive 30.9%. To summarize this, Mantiply was giving up fewer baserunners and he balls put in play against him were hit on the ground and with less authority, which usually leads to more outs. While the improved peripherals did not necessary show up in opponent wOBA, which clocked in at .313, there was some potential to see if he could sustain it over a full season in 2022.

While the season is only about 30% of the way through, we’re seeing even more improvements from Mantiply. The first take away is that Mantiply’s first pitch strike rate has drastically improved from 56.6% to 70%. Most likely that’s a result of him trusting the movement on his sinker and change-up to take him out of the middle of the plate. As a result of being able to jump ahead of hitters at such a prolific rate, it puts the hitter on the defensive and allows Mantiply to be able to lead them out of the strike zone with the late movement on his pitches. In 2022, Mantiply has induced a chase rate of 38.7% of the pitches he’s thrown outside the strike zone. That puts him in the 98th percentile amongst all MLB pitchers.

As a result of getting hitters to chase out of the zone, Mantiply has seen his ground ball rate elevate up to 58.5%. An improved ground ball rate usually bodes well for run prevention, as hitters have a batting average of .240 against ground balls and a slugging percentage of .265 in 2022. If a pitcher is particularly skilled at getting ground balls without the downside of hard-hit grounders, there is a legit BABIP suppression skill in play. Mantiply has allowed a BABIP of .264 on the season to date, which we can easily point to that 58.5% ground ball rate as the reason why. Combine that with a pitcher who’s only walked only one of the 70 hitters he’s faced on the year (1.4%) and you have a very difficult pitcher to string together offense against. The only way to beat Mantiply is to three together three singles, something that is very difficult to do against today’s pitchers.

What makes Mantiply so difficult is that he commands three pitches at an exceptional level. He’s more prone to use the curveball against left-handed hitters, although he still deploys it plenty against righties for stealing strikes on the arm side of the plate or going for the backfoot in strikeout situations. He uses his change-up almost exclusively to right-handed hitters, with 67 of his 72 change-ups being used against opposite-handed hitters. What makes his change-up so effective is it has the same movement profile as his sinker but with a 7.5 MPH drop-off.

As a result of his change-up looking similar to his fastball out of the hand, he’s gotten right-handed hitters to either take off-balance swings or completely miss the pitch outright. As a result batters are out in front and unable to put the barrel on the ball. Hitters have an average exit velocity of 81.9 MPH and launch angle of -4° against the pitch. Unsurprisingly that’s lead to opposing hitters batting .179 against Mantiply’s change. The curveball also tunnels fairly well with the sinker and change-up, taking almost an opposite direction break and something to attack hitters to the glove side of the plate. That pitch gives him a swing and miss weapon either to lead left-handed hitters out of the zone away or attack the backfoot of right-handed hitters. The overall repertoire gives Mantiply the ability to go for both ground balls and swings and misses, depending on the situation.

With the curveball being the main out pitch against lefties and a terrific change-up against righties, there aren’t any major platoon splits that we’d normally be worried about for left-handed relievers. Right-handed hitters are batting .190/.209/.190 against Mantiply, with left-handed hitters not faring much better with a .222/.222/.222 slash. With no major platoon splits against handedness, that gives Lovullo the freedom to deploy Mantiply in critical situations and not worry about match-ups although it’s much more preferred to use him in an inning where the majority of the scheduled hitters are left-handed.

Between June 20, 2021 and today, we have a 186 PA sample of Joe Mantiply being a quality left-handed reliever. The longer of a stretch we see of this run of dominance, the more confident I’m in that Mantiply has taken the next step to be a quality arm in the back-end of the bullpen for good. We’ll see how well Mantiply finishes out the season, as the team can control him through the 2026 season. Between now and then, we’ll take it year by year because you can expect volatility from most relievers. However if there is one arm in the D-backs bullpen best suited to weather that type of volatility, it’s Joe Mantiply.