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The Rays have another good reliever

Diego Castillo had a very impressive under-the-radar rookie campaign last season.

Toronto Blue Jays v Tampa Bay Rays Photo by Julio Aguilar/Getty Images

A few days ago over at FanGraphs, Jeff Sullivan pointed out a somewhat under-the-radar season put out by Rays setup man Jose Alvarado. Instrumental to the success of the famous 2018 Rays pitching staff, he, along with many other undetected names (Ryne Stanek, Chaz Roe, Hunter Wood, Yonny Chirinos) helped pieced together one of the most unique strategies the game of baseball has ever seen.

Intentionally not mentioned along with the names above for the sack of this article was Diego Castillo, a young right-handed reliever. His debut campaign with the Rays was one that deserved plenty of attention. It all started out in Triple-A for the Dominican Republic native, where he worked exclusively and versatility as a reliever. He over-matched hitters, pitching to the tune of a 1.03 ERA in 26 13 innings with strong peripherals to back it up.

His outlook as a major leaguer had some warts, but the reports were tantalizing.

“Castillo pitched on back-to-back days just once in 2016, which is potentially problematic for a relief prospect. When Castillo did throw (sometimes with four- or five-plus days of rest between appearances) his stuff was good. He was 96-100 with his fastball in the Arizona Fall League with a short but sharp upper-80s slider in the 84-88 mph range. He also throws a low-90s changeup, which is below average. That said, I’ve seen Castillo throw it to start off at-bats, allowing patient hitters to watch 91 go by for strike one only to blow 98 past them on the following pitch. He’s a middle-relief prospect on stuff, but the way Tampa has handled his workload gives me pause.”

Castillo kicked his major league career off in good fashion, posting a 1.88 ERA for the month of June. He slumped in July, flashing control issues (6.9 BB/9), but returned to an even more dominant form for the months of August and September (3.00 and 1.72 ERA for the respective months).

All in all, Castillo turned in a fine rookie season (3.18 ERA, 3.30 FIP, 1.0 fWAR in 56.2 innings), with peripherals that suggested more could be in store for the future (29.3 percent K-rate, 45.3 percent GB-rate). Diving in deeper, only eight pitchers averaged a higher velocity on their fastball (minimum 50 innings). Only 15 averaged a harder thrown slider. Baseball Savant, which marks his slider as a cutter, had batters hitting for a .106 average against the offering with a .174 xwOBA, which only trailed James Paxton.

Top Cutters in 2018

Rk. Player xwOBA
Rk. Player xwOBA
1 James Paxton 0.171
2 Diego Castillo 0.174
3 Jace Fry 0.209
4 Lou Trivino 0.218
5 Ryan Tepera 0.231
6 Jesse Chavez 0.232
7 Blake Treinen 0.235
8 Bud Norris 0.241
9 Roberto Osuna 0.247
10 Drew Rucinski 0.248
Minimum 100 results Baseball Savant

Pair that with a fastball that is essentially impossible to teach in terms of velocity and spin rate.

On a projection basis, the future outlook for Castillo looks great. Per Steamer, only 12 relievers are projected for a lower FIP than Castillo in 2019. A full season of performance at that level would be tremendous for the Rays playoff hopes next season.

The workload ability for Castillo still seems like the biggest question. But if he does indeed blossom into one of the better relievers in baseball, as the projections say he will, the Rays will have added a big boost to their creative pitching staff.


Patrick Brennan loves to research pitchers and minor leaguers with data. You can find additional work of his at Royals Review and Royals Farm Report. You can also find him on Twitter @paintingcorner.