One of the big questions in Cincinnati (and in fantasy circles, too) last April was, "What happened to Luis Castillo?" After an electric rookie start in summer 2017, the righty was carrying a 7.85 ERA and a garish 1.9 HR/9 rate through the season’s first month.

But a year later, the Reds’ ace of the future looks positively dominant once again. 

Castillo takes the mound in San Diego tonight sporting a 1.46 ERA, 2.33 FIP and a 33.3% strikeout rate, and the metrics fueling those numbers suggest staying power. He’s all over the Statcast starting pitching leaderboards: Second in whiff-per-swing rate, third in ground-ball rate, 13th in hard-hit rate and he’s yet to permit a single barrel (the most potent batted ball based on exit velocity and launch angle), let alone a home run.

Put all those factors together, and Castillo has looked like one of MLB’s best starters out of the gate. In fact, Statcast’s expected weighted on-base average (xwOBA, based on quality of contact and strikeouts) places Castillo second behind reigning American League Cy Young winner Blake Snell.

Lowest xwOBA, SP, 2019
Min. 50 batters faced (132 SP)
1. Snell: .205
2. Castillo: .220
3. Collin McHugh, Astros: .224
4. Tyler Glasnow, Rays: .235
5. Max Fried, Braves: .253

Castillo’s heater was one of MLB’s 10 fastest among starters last year, so his velocity didn’t go away. The difference from a year ago might lie in Castillo’s commitment to his changeup -- not only to throwing it, but also to consistently dangling it outside the strike zone. Left-handed batters feasted on Castillo last year, slashing .289/.351/.530, and a righty like him needs his changeup to neutralize the platoon advantage. Castillo’s mid-80s changeup can be devastating when tunneled off his mid-90s two-seam fastball, particularly when tailing away from a lefty’s power zones. Curtis Granderson got a first-hand look at that action while striking out on April 9.

The changeup was effective for Castillo last year (.200 BA against, 43.1% whiff-per-swing rate), and he threw it more than a quarter of the time. But he wisely began throwing the change more after last year’s All-Star break and hasn’t looked back; he's nearly made it his primary pitch to begin 2019 -- and has leaned on it in two-strike counts to produce some outstanding results.

Highest putaway rate, single pitch types, 2019
Min. 25 two-strike pitches thrown
1. Castillo (CIN), changeup: 47.2%
2. Edwin Diaz (NYM), four-seamer: 46.2%
3. Chris Archer (PIT), slider: 44.7%
4. Jeff Samardzija (SF), slider: 43.3%
5. Luke Jackson (ATL), slider: 42.3%
Putaway rate: % of two-strike pitches resulting in a strikeout

Hitters haven’t had success against that pitch in any count, really, as Castillo’s change-of-pace has held them to just four singles and 25 strikeouts in 41 at-bats. Part of that dominance might be from where Castillo is placing the pitch:

A hitter can’t routinely inflict much damage on a pitch that low, and that’s the point here. As Baseball Prospectus’ Zach Crizer noted Thursday, Castillo isn’t giving opponents much to hit, period, while posting MLB’s fifth-lowest rate of pitches in the strike zone. Castillo’s 13.5% walk rate is probably higher than he and pitching coach Derek Johnson would like it to be by season’s end, but in the meantime, he’s playing an effective waiting game with overzealous hitters. Hitters can adjust to most pitchers when they know they aren’t throwing strikes. But it’s likely more difficult to lay off against Castillo’s changeup when it looks so similar to his two-seamer before darting out of the zone. The pitch is simply too tantalizing, and the numbers bear that out; opponents have chased half of Castillo’s out-of-zone changeups, the Majors’ third-highest rate on that pitch type.

Pair that change-of-pace with Castillo’s two different fastballs, each coming in with premium velocity, and it can be a nightmarish guessing game.

"The changeup is his best pitch," Brewers manager Craig Counsell said Monday, after Castillo struck out eight Milwaukee hitters, including six on the changeup. "It's obviously paired with big velocity. When you're at 96 when you want it, it makes the hitters start. That's why velocity is so valuable.”

"The difference is, he throws it to right-handed hitters, which you don't see a lot,” Counsell added. “That's what makes him a little bit unique, that he's throwing changeups to the same-sided hitters."

Righties or lefties, it hasn’t mattered who’s stepping into the box against Castillo so far. The pitcher who showed so much promise two summers ago has identified his best pitch, and like so many hurlers leaning more and more on offspeed in 2019, he looks like he’s willing to go to the well again and again.