Do the Diamondbacks Have Enough Left-Hand Reliever Depth?

As one veteran surprises, another falters, and the depth behind them has question marks

Mar 30, 2025; Phoenix, Arizona, USA; Arizona Diamondbacks pitcher Joe Mantiply (35) on the mound to pitch in the eighth inning against the Chicago Cubs at Chase Field. Mandatory Credit: Allan Henry-Imagn Images / Allan Henry-Imagn Images

The Arizona Diamondbacks were very fortunate to land Jalen Beeks at the end of spring training. Released by the Houston Astros on March 22, he signed with the D-backs March 26 and has stepped right in to provide quality innings from the left side.

In six games he's thrown 9.1 innings, having recorded five or six outs in four of those outings. He's allowed just one run on on three hits, two walks, a homer, and eight strikeouts. While his 0.96 ERA is not sustainable, his 3.30 FIP (Fielding Independent Pitching) and 1.27 xERA indicate he's pitched very well.

As good as Beeks has been in the early going, it needs to be remembered he's essentially been a league average reliever for most of his seven year career, posting a 4.31 ERA in 357 innings, or 98 ERA+.

The primary set up lefty in the bullpen is A.J. Puk. He was phenomenal in 2024 when he came over from the Marlins after a trade deadline deal, posting a 1.32 ERA in 27.1 innings. So far this year he's looked a little less dominant, having given up two runs on two solo homers in five innings. But he's struck out seven, walked just one, and stuff wise, he looks the same as last year.

Beyond these two lefties, things are not looking quite so secure, however. Long-time veteran reliever Joe Mantiply is struggling, and it appears that it's not just the vagaries of small sample size with relievers. In 5 games, 4.1 innings, he's allowed four runs on six hits, including a homer. He hasn't walked a batter and struck out four. He has an 8.31 ERA and a 4.13 FIP.

Mantiply has a career 4.10 ERA versus a 3.17 FIP. It's fairly typical for him to have a higher ERA than the run estimators indicate. That's because he doesn't walk too many, gets his strikeouts, and usually doesn't give up many homers. But the one thing he's always done is give up plenty of base hits, about one per inning throughout his career. (His H/9 is exactly 9.0)

Mantiply has never been a hard thrower. He's dependent on his low arm angle and ability to locate and keep the ball down to get outs. But sometimes there are tipping points when it comes to velocity loss that simply result in a pitcher becoming too hittable, and Mantiply may have reached that threshold.

Mantiply's sinker usually sat at 90-91 going back to his All Star season in 2022. That was enough, considering the movement and arm angle, to set up his curveball and change up. Since then there has been a decline every year in the velocity in all his pitches, and the sinker is down to averaging 87 MPH in the early going of 2025.

Joe Mantiply Velocity Chart since 2019 / Baseball Savant MLB.com

It's reasonable to question whether Mantiply will be effective going forward. If he's not, then the D-backs will continue to lean heavily on Beeks, and start moving him into more high leverage innings.

If Mantiply continues on his downward trajectory and Beeks regresses to the mean, the D-backs could quickly find themselves searching for left-hand help in the bullpen.

The depth in the minor leagues is lacking. Kyle Nelson was optioned out to Triple-A Reno on March 19, and then assigned to the Arizona Complex League on March 28. Coming off Thoracic Outlet surgery last year, Nelson did not look sharp in spring.

Veteran Jose Castillo was thought to be a potential option early on in camp, as his stuff was looking very sharp. But the oft injured pitcher suffered a groin injury fairly early in camp, and he too has yet to appear in a game for the Aces, starting the year on 7-day IL.

Kyle Backhus and Jake Rice are the two left-hand relievers currently pitching in the Reno Aces. Neither has ever pitched in the Major Leagues. Backhus is 27 years old, and similar to Mantiply, depends on a low, almost submarine arm angle with his 90-91 MPH fastball. He throws a 79 MPH slider as well.

There just aren't a lot of left-hand options in the system. At some point they might consider to convert Tommy Henry to relief. He continues to be hit very hard as a starter toiling in Triple-A. But his stuff may not profile well as a reliever. Still, at some point the D-backs will have to try if they are to get any value out of Henry.

One outside possibility would be to sign veteran left-hander Drew Smyly to a minor league contract if he were willing. But he would need at least four weeks or more to get ready, as he's not been in a major league camp. Since he never signed this offseason, he may not be willing to go the minor league contract route.

Summary

With A.J. Puk the D-backs have a strong left hand setup and sometimes closer option. Beyond that however, the longer term options from the left side are going to continue to be Beeks and Mantiply for the foreseeable future. Whether Beeks can maintain his early-season success, or Mantiply can find effectiveness despite what looks like irreversible velocity loss, remain to be seen.

Beyond those two, there are no internal options available to the team that are either healthy or have major league experience. Watch this space for further developments on this situation.