Diamondbacks 2025 Player Projections: Joe Mantiply

The D-backs' reliable left-hand reliever is projected to have a solid 2025.

Jul 9, 2024; Phoenix, Arizona, USA; Arizona Diamondbacks pitcher Joe Mantiply against the Atlanta Braves at Chase Field. Mandatory Credit: Mark J. Rebilas-Imagn Images / Mark J. Rebilas-Imagn Images

This article is part of a series reviewing the projections for each member of the Arizona Diamondbacks 40-man roster, as well as select prospects and non-roster invitees. The order of presentation was chosen using a random number generator.

The projection is comprised of the average of ZiPS and Steamer rate statistic projections, which can be found on FanGraphs. The playing time projections are our own, based on our estimates. As the D-backs make further moves throughout the offseason, the playing time projections will be updated, and relevant counting stats such as home runs and WAR can be updated.

LHP Joe Mantiply, 33

Joe Mantiply statistics and 2025 projection / Alex D'Agostino, Diamondbacks ON SI

Why Mantiply might outperform this projection

Left-handed reliever Joe Mantiply has been one of the Diamondbacks' most reliable bullpen arms. The former All-Star nearly matched his career-high in innings in 2024, and pitched to a solid 3.92 ERA.

But Mantiply's strengths are oftentimes unrecognized, below-the-surface factors. Certainly, a 3.61 ERA in 2025 would be an above-average number, but that projection likely doesn't fully account for the pitcher that Mantiply truly is.

I wrote about this in detail during the 2024 season: Mantiply's surface ERA doesn't reflect his worth in Arizona's bullpen.

Related Content: Joe Mantiply's Numbers Might Not Show True Worth

In fact, the lefty pitched to a 3.92 ERA in 2025. That alone is an above-average number, and one that can be of use to most teams' bullpens. But it didn't tell the full story. Mantiply posted a stellar 2.51 FIP and 3.24 xERA to go along with that raw ERA.

And it wasn't an easy workload, either. Facing the likes of Shohei Ohtani, serving as an opener on occasion, and getting some of Arizona's toughest outs proved that the southpaw is capable of high-pressure performance.

So when examining his 2025 projections, it's easy to misinterpret the 3.61 ERA as a notable improvement from 2024.

Sure, the raw numbers are different, but if his batted ball luck and raw results manage to correlate more closely in 2025, he could easily smash his projected ERA, even getting down close to the sub-3.00 range.

His projection is anything but unreasonable, and his rate stats seem to closely mirror what he's done consistently over his career, but it's quite likely the ERA will see a big boost if he pitches even similarly to 2024, with a chance for significant overperformance in overall raw results.

Why Mantiply might underperform this projection

As with any relief pitcher, volatility is their worst enemy. Mantiply is no stranger to that principle. Despite being a very consistent arm, a mere handful of multi-run outings ballooned his ERA in 2024.

Since Mantiply doesn't have the highest velocity, his value comes from his unique delivery and arm angle, as well as pure movement of his stuff. That's been quite effective as a whole, but the slightest loss of command or movement can easily lead to disaster.

He's exceptional at garnering chases and ground balls, but when that ability dips, he tends to get hit very hard. A couple of poor outings here and there, and his overall numbers could skyrocket above his solid projections.

Summary

There's no true reason to expect a poor season from Mantiply. He's been a solid reliever, an All-Star, and generally better than his numbers have shown. Plus, with the 2024 mid-season addition of A.J. Puk, some of the left-handed workload is lifted from his shoulders.

There's always a potential for volatility in relievers, and his stuff can be quite hittable when not placed properly, but as a whole, it's seems more likely he'll hover around his projections or perhaps below, despite him entering his age-34 season.

If Arizona does manage to add another left-hand reliever, Mantiply could slot in comfortably to a somewhat reduced role, while still providing quality, important innings for the Diamondbacks in 2025.

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Arizona Diamondbacks 2024 Player Review: Joe Mantiply