Pittsburgh Pirates Prospects: Diego Castillo’s 2021 Breakout


The Pittsburgh Pirates acquired middle infield prospect Diego Castillo at the trade deadline this year, but is his breakout season for real?

The Pittsburgh Pirates made a handful of moves at the trade deadline. One of the moves that didn’t get that much attention throughout the greater baseball world was Clay Holmes to the New York Yankees for two infield prospects. One was Hoy Park and the other was Diego Castillo. Castillo had a huge breakout season in 2021, but is his production for real?

Now going into this season, Castillo was never a big hitter. From 2017 through 2019, his highest wRC+ in a single campaign was just 93. He rarely struck out but didn’t walk a whole lot either. It also didn’t help that he had absolutely no power. He didn’t even put up an isolated slugging percentage above .100 from 2016 through 2019.

But Castillo started the year looking like a changed player. He began the campaign out at the Yankees’ Double-A affiliate where he batted .277/.345/.504 with a .367 wOBA, and 129 wRC+. Castillo kept around his trademark low strikeout rate, going down on strike three just 13.7% of the time.

Castillo increased his walk rate to a solid 8.4% mark. He was also hitting for a lot of power. Castillo had a .228 isolated slugging percentage and 11 home runs in 249 trips to the plate. He had already beaten his career home run total from 2015-2019 in less than 250 plate appearances.

His strong performance kept up post-trade. After he was acquired by the Pittsburgh Pirates, Castillo was sent to Double-A Altoona where he had a .282/.342/.445 line, .343 wOBA, and 114 wRC+. Castillo’s power did take a step back with a .164 ISO, albeit in only 114 plate appearances, but he kept up a very similar walk rate (8.3%). But the big improvement was getting his strikeout rate below 10%, coming in with a mark below his walk rate of 7.4%.

At the end of the campaign, the Pittsburgh Pirates promoted the infielder to Triple-A Indianapolis where he raked in a small sample size of 70 plate appearances. Castillo put up a .278/.414/.500 line, .400 wOBA, and 146 wRC+. His ISO reached well above .200 once again, posting a .222 mark. He also had the same amount of walks to strikeouts of 13. That comes in with an 18.6% rate. That is a bit high of a strikeout rate for him, but given that he, overall, massively improved his walk rate this year, I wouldn’t be very much worried about him.

So yeah, he had a huge breakout campaign. He ended the season with a .842 OPS when he never had an OPS above .700 in more than 300 plate appearances before. But was his breakout something that can be counted on?

Well for one, Castillo didn’t get overly lucky on batted ball luck. Castillo had a batting average on balls in play (BAbip) of just .282 overall throughout 2021. That’s not very far off from his overall batting average in 2021 of .278. Just a .004 point difference. It also doesn’t deviate by a vast amount from his career BABIP going into this year. From 2015 through 2019, he had a .284 mark. So in terms of this, it’s likely sustainable.

What about batted ball rates? That’s where we’re going to see the most deviation from the path of his former self. Castillo was consistently a high-ground ball hitter. He usually sat around a 48%-50% ground ball rate. Meanwhile, he only ever once had a fly ball rate above 35%, that being in 2019 with a 41% rate.

This year, however, he kept his ground ball rate below 40% (except at Triple-A, but overall having a low ground ball rate), while posting a strong flyball rate. He was hitting a ton of balls in the air. Castillo had a fly ball rate of at least 37% at each level and minor league affiliate he played for this season. Now granted, he did get a little more pull happy. But as indicated by his BAbip, it didn’t seem to hurt him all that much when it came to getting hits.

Currently, FanGraphs pins him as a 40+ FV prospect. FV is far from a perfect tool to evaluate players. FanGraphs isn’t nearly as bullish with their FV grades as some with only 29 total players that project to be better than average (55 FV at the very least) and only 112 projected to be at least average (50 FV at the very least). The chances that only 29 prospects end up as above-average players at the very least is low, so it’s good to give context to FanGraphs’s grades. Still, a 40+ FV isn’t bad.

According to the site, they would have given him a 45 FV had it not been for his age. That would put him among the likes of Bubba Chandler, Brennan Malone, Hudson Head, Ji-Hwan Bae, and Cody Bolton. Not a bad group of prospects to be compared to. Chandler has the potential to be a quality two-way player.

Malone was a first-round pick a couple of seasons ago. Head is a highly athletic outfielder with a very high ceiling. Bae had a quality campaign at Double-A. Bolton is the only one who truly has a good chance of getting a downgrade during next year’s rankings.

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Is Castillo’s production just a fluke, or is it truly a breakout season? His approach at the plate, as well as hitting the ball in the air more often, and strong ability to make consistent contact are all things that work in his favor. While I don’t think he’s going to be a consistent 130-140 wRC+ guy like we’ve seen him be this year, I don’t see why he can’t be a solid 105-115 wRC+ batter with a little bit of pop if his fly ball rate remains high.