Going into 2022, what we could expect from Diego Castillo was something of a question mark. After arriving in Seattle via trade mid-season 2021 the results were rather mixed, and of a very small sample size due to some time on the injured list. There was some fear that with a declining velocity his results would decline as well, especially given that his strength was in punching batters out with his slider. Since then the experience of watching him pitch has often been compared to former Mariner Fernando Rodney, in both a positive and negative way. After a full season of results we have a much better picture of how Diego will contribute to the team, but some of the answers require some reading between the lines. The short answer is that he isn’t the sword master he once was, but the way he is inducing contact, and when he chooses to step his game up, are still of great value to his role on the team.
First, a look at how he stacked up compared to the rest of the league.
Traditionally, Diego’s bread and butter has been strikeouts. In 2021 he posted a career high K-BB of 24.9%. In 2022 there was a steep drop in his K% and a slight rise in his BB%, leading to a much lower K-BB of 14%. We can see in the graphic above that amounted to an about average strikeout rate and a walk rate on the worse end of things. His total strikeouts were still decent though at 53 in ‘22, and projected by Steamer to bounce back to 67 in ‘23. Less strikeouts and more contact, with career low SwStr% and CStr% of 11.9 and 15.4 respectively, but still in the upper echelons of the league in categories of HardHit% and xwOBA.
We can likely point to his overall location as being at least partially responsible to some of his backwards movement. Since he is primarily using a two-pitch arsenal, location is key to his success.
That was his location for his slider and sinker in 2021.
And there is his slider and sinker location in 2022, with his slider in particular finding the edge of the zone significantly less. It certainly is a cause for worry, and a fall from form given his history. When you throw your slider two-thirds of the time, you need to be able to entice hitters into chasing it, and not leaving it hanging over the heart of the plate like he has been.
This regression in location certainly led to his drop in strikeouts and increase in contact, but much of that contact was manageable. On top of that, Castillo seems to have benefitted from pitcher friendly T-Mobile Park and possibly last year’s typically deader ball, seeing around career average numbers for ground ball and fly ball rates, but achieving a career low home run to fly ball rate of 10%.
The fears of his regression were somewhat justified, although he saw a slight uptick in his velocity of all his pitches compared to 2021, so that trend has leveled out for the time being. One notable area of regression (aside from his K-BB%) were in his ERA- and FIP-, stats that, much like wRC+, compare to a league average of 100, but conversely the lower the number is better. In 2021 his ERA- was 67, and his FIP- was 90. In 2022 his ERA- was 99 and his FIP- was 96, both just slightly above average. It’s not all bad though. While his actual FIP of 3.68 compared near equally to his xFIP of 3.85, his actual ERA of 3.64 was inflated compared to his xERA of 2.96. None of these numbers are world-shatteringly good, but neither are they abysmal. Diego Castillo may not consistently be in the same form that made him a frequent feature on Pitching Ninja a few years ago, but it’s not completely gone either. Only two of his walks were on 4-0 counts, and he mostly came through in his 3-2 counts.
So what does this mean for the Seattle Mariners? Luckily, Seattle won’t be relying on him to be lights-out, closer material. For that they will have Paul Sewald and Andrés Muñoz returning, as well as Matt Brash if he doesn’t find a spot in the rotation or a trade package. They will need Castillo to provide value though, especially with the loss of Erik Swanson. Last year the M’s bullpen ranked 13th league wide in fWAR for a total of 4.1, of which D. Castillo contributed 0.3. The 2023 Steamer projection for him is an fWAR of 0.4, a marginal improvement. If his projections prove true we can expect about the same overall numbers this next season. His value doesn’t lie in his overall numbers, or in him being a star reliever. It comes from his five seasons of experience, and that he knows how to turn it on and come through in the clutch. Clutch is more than just a euphemism though, it’s an actual stat.
As noted above, clutch is a stat that compares a player against himself, so how useful that is relies heavily on how good a player is or isn’t on average. As we discussed already, Diego has trended closer to league average, but he’s much better than that when it matters. His total WPA contribution last year was 19th best in the league among relievers at 2.32, leading Mariners’ relievers, and his Clutch was a 0.99, good for 7th best in the league among relievers, or 16th best among all pitchers with a minimum of fifty innings pitched. This 9th inning last year against the Mets is just one example of that in action.
With this in mind, maybe the Fernando Rodney, edge-of-our-seat when the game on the line comparison is a little unfair. Diego Castillo seems to be a great personality fit on the club, and has a whole season with the team under his belt to feel acclimated to the team and the ballpark. Perhaps most importantly though, his edge sharpens when the battle calls for it.