Miguel Castro Is a Great Fit for Arizona’s League-Worst Bullpen

On December 2, the Arizona Diamondbacks came to terms with 27-year-old free agent reliever Miguel Castro on a one-year, $3.5 million contract. I wouldn’t blame you if you failed to catch the news amidst the flurry of moves and rumors from the Winter Meetings; the Turners and deGroms of the world have stolen much of the focus. Nevertheless, while his signing was relatively small, I was drawn to write about Castro for several reasons. For one thing, he’s just a lot of fun to watch. I mean, tell me you disagree:

Standing 6-foot-7, Castro is one of the taller pitchers in baseball, but his listed weight is only 205 pounds. To put that in perspective, he’s the same height as Aaron Judge but with about 75% of the body mass. Yet despite his slim frame, Castro throws high heat in an effortless fashion, whipping his long limbs around to sling a sinker at 98 mph. The length of his extremities also allows him good extension on top of his velocity. To finish it off, he tends to complete his delivery with a bit of extra flair on his back leg kick. Simply put, it’s a unique and satisfying pitching motion — the slime ASMR of windups, if you will.

I also enjoy watching Castro’s changeup, which has consistently been one of the fastest in baseball. Ben Clemens wrote about this particular pitch a couple years back, and it remains one of my favorite changeups to watch. You just don’t often see an offspeed pitch top out above 95 mph:

Beyond his distinct aesthetic appeal, I was also drawn to Castro for the opposite reason: he’s not unique at all. He’s a right-handed middle reliever, which there always seems to be an abundance of each offseason. As a group, middle-inning relief pitchers are the most widely sought-after players on the market. Not every team needs (or manages to net) an Aaron Judge or Carlos Rodón, but everyone is looking for inexpensive bullpen help, year in and year out. That means, funnily enough, the market was arguably bigger for a pitcher like Castro than it was for any of the marquee free agents. He could have signed almost anywhere, so it’s particularly interesting to see where he — and other free agents like him — ultimately end up. Trea Turner joining the Phillies and Jacob deGrom heading to Arlington were both on my offseason bingo card. But who among us would have predicted Miguel Castro to sign with the Diamondbacks?

In hindsight, however, it’s actually a lovely fit. The D-backs had one of the worst bullpens in baseball last season – perhaps the very worst. They finished at the bottom of the league in WAR, and things only got worse as the season went on. Their 5.05 xFIP in September/October was the worst by any team in any month all season. The driving force behind their utter ineptitude was a lack of strikeouts; Arizona’s bullpen finished dead last with 7.71 K/9, well below the league-average rate of 9.02. It’s a major reason why they had so much trouble stranding runners, finishing with the fourth-lowest left on base percentage in baseball.

Castro can lend a hand in that department. He would have led the Diamondbacks bullpen in strikeout rate last season, and he’s averaged 9.79 K/9 over the past two years — more than any D-backs reliever (min. 20 IP). Steamer projects him to lead the team in total strikeouts in 2023, with 66 punch-outs in 64 frames. Castro’s biggest weakness, meanwhile, is Arizona’s one strength: free passes. Okay, “strength” might be too strong a word – the Diamondbacks’ bullpen didn’t do anything well in 2023 – but they finished with 3.60 BB/9, only a tad higher than the league-average figure of 3.50. Castro, on the other hand, finished with 4.66 BB/9 this season, just a year after allowing a whopping 5.50 BB/9 in 2021. Steamer projects him to issue more free passes than any of his bullpen mates in 2023.

The Castro signing gives manager Torey Lovullo another arm he can call on when he needs a strikeout or if Arizona is facing a strikeout-prone lineup. Alternatively, in a situation where a walk would be extra detrimental, Lovullo already has several better options to choose from. To that point, Castro brings additional skills to the table to help his manager play better matchups with his bullpen: high velocity and a high groundball rate. The D-backs had one of the slowest-throwing bullpens in the majors last year. Their relievers averaged just 92.6 mph on four-seam fastballs — and that was their fastest collective pitch. Castro, meanwhile, averaged just under 98 mph with his sinker, topping out at 100 mph. He instantly becomes the hardest thrower in his new bullpen:

With that sinker, Castro also induces a ton of groundballs. Over the past two years, more than half the balls in play he’s given up were hit on the ground. By contrast, the D-backs bullpen had a 38.1% groundball rate last season. Only one of their relievers, All-Star Joe Mantiply, finished with a groundball rate above 50%. Neither low velocity nor a low groundball rate is necessarily a problem, but it’s never a bad idea to diversify the skillset of your relief corps. In Castro, the Diamondbacks add a fire-powered arm who can induce groundballs better than any other right-hander on the roster.

The suitability of this deal doesn’t end there, and this signing isn’t a one-way street – just as Castro is a good fit for the Diamondbacks, the Diamondbacks are a good fit for him. For instance, Castro could benefit from studying under pitching coach Brent Strom. Already famous for his work with the Astros staff from 2014-21, Strom did a fantastic job with D-backs relievers Mantiply and Kevin Ginkel in 2022, each of whom has something in common with Castro. Ginkel is the more obvious comp; he’s a hard-throwing, 28-year-old righty who struggled with control and had terrible platoon splits before his breakout. Mantiply is a soft-tossing lefty, but he has the same pitch mix as Castro: a sinker, a slider (which he uses against same-handed batters), and a changeup (which he uses against opposite-handed batters). Mantiply and Ginkel both struggled to reach their potential before working with Strom.

Moreover, the D-backs pitching coach is especially well-suited to work with Castro because of the right-hander’s dwindling spin rate. During Strom’s tenure in Houston, the Astros took spin rate very seriously. They targeted pitchers with good spin and helped in-house arms boost their RPM. Some of those boosts might have owed something to sticky stuff, but nevertheless, Strom knows the value of a high spin rate and what it means to work on that aspect of the game. Castro, strangely, saw his spin rate plummet during the 2022 season. Take a look at the spin rate on each of his pitches in 2021 and then this season:

It’s important to note that Castro doesn’t appear to have been a victim of the league’s sticky stuff crackdown; his RPM actually went up a bit after the new rules took effect last summer. However, the difference between his spin rate in September 2021 and April ’22 is stark. It didn’t seem to affect his results, but it’s a worrisome sign all the same. Castro doesn’t have very deceptive stuff, and the lower his spin rate, the more hittable his pitches — particularly his sinker — will be. Time will tell if Strom can figure out the problem and help Castro recover his missing RPM.

Castro could also benefit from working with Arizona’s infield defense — or, more accurately, from one particular infield defender. Castro induces his fair share of balls on the ground, and a disproportionate number of those are hit by left-handed batters. Over the past two years, lefties have put the ball in play in 64.3% of their plate appearances against Castro. (In contrast, righties have only done so in 51.6% of PA.) Of all those balls lefties hit into play, more than half were on the ground. And of all those groundballs, nearly half were pulled to the first base side. With that in mind, the new infield shift rules could harm Castro more than most — and that’s where the Diamondbacks come in. Arizona has the best defensive first baseman in the sport manning the far-right side of the infield dirt. Christian Walker led all first basemen with 14 OAA in 2022, and he excelled on plays to his right — the kind of plays that might be made by a shifted second baseman or shortstop. With Walker behind him at first base, the new shift rules might hurt Castro a little less.

Before I wrap up, I feel the need to clarify that Castro won’t move the needle that much in the grand scheme of things. I’ve spent 1500 words hyping him up, and I wouldn’t want my nerdy enthusiasm for a middle reliever with a pleasing delivery to be mistaken for more than what it is. At the end of the day, Castro is an unexceptional bullpen arm with 0.2 career WAR, coming off a season in which he missed 12 weeks with a shoulder strain. He’s not Edwin Díaz or Kenley Jansen. With that caveat out of the way, though, I still see this as a smart signing for both player and team. Castro brings some new and valuable tools to Arizona’s bullpen, and in return, he gets to work alongside a couple of helpful new colleagues. Adding Castro isn’t going to transform the Diamondbacks into contenders, but it represents a step in the right direction toward fixing the league’s worst bullpen.