Although the Mets suffered many blows to their cadre of long relievers, swingmen, and mop-up guys in 2021 due to injury, the back end of the Mets’ bullpen that handled innings six and seven and onward stayed remarkably healthy. The Mets had five qualified relievers in 2021: Edwin Díaz (62 2⁄3 IP), Trevor May (62 2⁄3 IP), Jeurys Familia (59 1⁄3 IP), Aaron Loup (53 2⁄3 IP), and Miguel Castro, who let the pack in innings pitched with 68 1⁄3 innings in 2021. Although there are both positive and negative things that can be said about Luis Rojas’ bullpen management in 2021, one thing he did do was spread the workload fairly evenly among these pitchers (and Seth Lugo as well, who fell just short of 50 innings due to the late start to his season, but was also in this group of heavily used relievers when he did return); all five of these pitchers appeared in between 63 and 68 games this season. And other than Loup, who was the obvious standout performer of the group, putting together his historic season, all of the other relievers put up remarkably similar numbers in 2021, including Castro.
The Mets acquired Castro last season in a trade deadline deal that sent Kevin Smith to the Baltimore Orioles, working to bolster the bullpen which was a source of weakness in 2020. A sinker/slider/changeup power righty with nasty stuff, Castro’s bugaboo has always been the walk. The Mets took a bet on his rising strikeout numbers and improved walk rate ahead of the 2020 trading deadline, hoping to get another middle-to-late-inning option in relief for multiple years to come.
And in his first full season as a New York Met, they have gotten that—mostly. Castro came out of the gate swinging (or inducing swings and misses, I should say) in April, maintaining the markedly improved strikeout to walk ratio he showed with the Orioles in August of 2020 (3.4 K/BB in April 2021). He gave up just two earned runs the entire month—a 1.64 ERA over that span. Specifically, he was seeing a lot of whiffs with his changeup in the early going. But by early July, Castro’s ERA had ballooned to over 4 and he struggled particularly with his control in the latter half of June, walking ten batters that month—the most of any month in 2021. This suspiciously corresponded with the crackdown on sticky substances and Castro was one of the Mets players mentioned most often in speculative conversations about who might be affected by MLB’s enforcement.
However, Castro exhibited none of the characteristic sharp drops in spin rate that were clear from other suspected sticky stuff users (specifically the ones going far beyond sunscreen and rosin concoctions to things like Spider Tack), although it was noted at the time that his changeup spin took a rather sharp tumble. But that seemed to level out by the end of the season when one looks at the stats in aggregate. Still, although it was his least-used pitch, it was one that he had success with in the early season and the fact remains that the sticky stuff crackdown may have had an effect on Castro’s ability to use that pitch effectively and/or to control his entire arsenal. There is no substantive proof that Castro was using sticky substances and he did settle down after his rough patch—either making the necessary adjustments or simply because of random in-season variation.
Castro went on to have a strong August before struggling again down the stretch. But then again, most of the team struggled down the stretch. All told in 2021, Castro pitched to a 3.45 ERA in 2021, holding opponents to a .189 batting average; both of these marks are career highs for Castro. But over the course of the entire season, his walk rate was still too high (5.50 BB/9) and his strikeout rate (9.85 K/9), while solid and an improvement over most of his time in Baltimore, did not match his elite high mark in 2020 (13.86 K/9). One thing Castro was fairly good at in 2021 was limiting home runs (0.9 HR/9), putting up a better mark in that department than either Jeurys Familia (1.52 HR/9) or Trevor May (1.44 HR/9), but not as good as Edwin Díaz (0.43 HR/9) or, of course, Aaron Loup (0.17 HR/9).
Advanced metrics like DRA (4.43) and FIP (4.36) indicate that Castro may have gotten a little lucky this year, but by ERA+ (116), he was still well above league average and in fact, 2021 was the second-best season of his career by that metric. Fangraphs WAR seems to really hate Castro (0.1 fWAR), but Baseball Reference (0.6 bWAR) and Baseball Prospectus (0.9 WARP) look more kindly upon his value in 2021. Although nothing about Castro’s 2021 season was extraordinary, there is something to be said for the value of ~70 innings of ~3.50 ERA performance out of the bullpen. The 2022 season is the 27-year-old Castro’s final year of arbitration before becoming eligible for free agency next offseason and he will almost certainly be tendered and serve a similar role in 2022 as he did this season.