Sunday was a rollecoaster for Reds righty Luis Castillo. At Great American Ballpark, he needed just 32 pitches to set the Giants’ starting nine down in order for the first time, faced the minimum number of hitters through five innings, and didn’t allow his first hit until he’d gotten one out into the sixth — all while staked to a four-run lead. Before he could escape the frame, however, he allowed a walk and two other hits, including a game-tying three-run homer by Buster Posey. Given that he’d thrown just 81 pitches to that point, manager David Bell sent him back out for the seventh. He put up a zero, and left with the game tied, but the Reds lost, 6-5. Bummer.

The outing cost Castillo his lead atop an NL ERA leaderboard that at first blush appears to be drunk, with guys like Kyle Freeland (5.90 ERA),Yu Darvish (5.79), Aaron Nola (5.06), and Noah Syndergaard (5.02, even after Thursday’s heroics) stumbling along while Zach Davies (1.56), Castillo (1.97, up from 1.45 after his previous start), Caleb Smith (2.00) and Jordan Lyles (2.20) shine.

A closer look at the 26-year-old Dominican’s numbers shows that his spot there is no fluke. Castillo has shaken off last year’s sophomore slump with a performance reminiscent of his tantalizing 15-start rookie season from 2017, asserting his spot among the majors’ upper echelon of starters. Prior to Sunday, he hadn’t allowed more than two runs in any start this season, a performance that earned him NL Pitcher of the Month honors for April. In 50.1 innings though Sunday, he’s second in ERA, strikeouts (59), groundball rate (57.8%), and WAR (1.4), fourth in strikeout rate (30.3%), fifth in home run rate (0.54 per nine), and sixth in FIP (2.89).

Recall that Castillo, who was originally signed out of the Dominican Republic by the Giants in 2011, and traded to the Marlins in 2014 and the Reds in 2016, was effectively treated as a lottery ticket in deals that respectively sent Casey McGehee and Dan Straily the other way. He was also dealt to the Padres and back again in a mid-2016 pair of trades that centered around whether San Diego had been forthcoming regarding Colin Rea’s elbow issues.

Though renowned for a fastball that could reach 101 mph, he cracked only one major prospect list, placing 94th on that of ESPN’s Keith Law in the spring of 2017. Law graded his changeup as plus but noted that his third pitch was a fringy slider. Baseball America, which placed him second on its Marlins list that same spring but still shy of its Top 100, praised his easy velocity, projected his slider as above-average, and noted that “he has a feel for a power changeup, but he’s still finding the right grip. It has the potential to be an average pitch as well.” Our own Eric Longenhagen, who had Castillo 10th on Cincinnati’s 2017 list, described the slider as flashing plus and the changeup as ” below average but there’s good arm speed here (that should be obvious, this guy bumps 100) and it could get to average with reps.”

Things came together for Castillo once he joined the Reds’ organization; his overall strikeout rate jumped from 19.8% in 2016 (21 starts at Hi-A, three at Double-A) to 25.6% in 2017 (14 starts at Double-A) before he was recalled on June 23, 2017. In 15 starts before being shut down following his September 6 outing, he pitched to a 3.12 ERA and 3.74 FIP while striking out 27.3%. His changeup had advanced, to say the least. Its average velocity of 87.5 mph (compared to his average four-seam fastball velocity of 97.6, second in the majors among pitchers with 80 innings) put him in the 85th percentile, and batters couldn’t do anything with it. They hit .138/.200/.207 while homering once and striking out 40 times on the 95 plate appearances that ended with the pitch. His .183 wOBA, .185 xwOBA, and 22.8% swinging strike rate on the pitch all placed him in the 94th percentile or better among pitchers who threw at least 100 of them (he threw 334 of them; his 22.7% usage rate itself was in the 89th percentile). As for his slider, which he threw 15.2% of the time, batters hit just .093/.111/.116 while striking out 17 times on the 45 PA that ended with the pitch, and whiffed 15.2% of the time on it.

Last year, Castillo was hit hard early, yielding six homers apiece in April, May, and June. He carried a 5.49 ERA and 4.77 FIP into the All-Star break; he wasn’t great in August either, but salvaged the season with a stronger second half (2.44 ERA, 3.61 FIP) en route to a 4.30 ERA and 4.32 FIP overall, (105 ERA-, 104 FIP-). Not much to write home about.

All of which makes this year’s results so much more reassuring. It’s early yet, and the sample sizes are accordingly small, but a few things stand out. First, he’s using his changeup with greater frequency (31.3%, third among 94 qualifiers), and the results on the pitch are much more in line with 2017 than with last year. Hell, they’re devastating:

Luis Castillo’s Changeup Year PA AVG OBP SLG wOBA xwOBA EV GB% O-Swing% Zone% SwStr% 2017 95 .138 .200 .207 .183 .185 83.2 60.4% 45.0% 36.8% 22.5% 2018 242 .200 .227 .330 .239 .211 84.5 52.8% 52.8% 38.3% 25.9% 2019 85 .104 .188 .104 .148 .155 81.8 66.7% 54.5% 26.8% 30.5% SOURCE: Brooks Baseball, Baseball Savant wOBA, xwOBA and EV via Baseball Savant, all other stats via Pitch Info.

Castillo is throwing his changeup in the strike zone with less frequency than ever, yet batters continue to chase it, and they’re swinging and missing at their highest rates to date. When they do connect with it, they’re generating a lot of weak contact, usually on the ground. I couldn’t fit his hard-hit rates on the pitch into the overstuffed table above but thus far, batters have produced exit velocities of 95 mph or higher on just four out of 33 batted ball events (12.1%), compared to rates of 27.1% in 2017 and 31.3% last year. Of those four events, two were singles and two were groundouts. Nobody has homered off the pitch this year, and just one batter did in 2017 (Jake Lamb on July 20), compared to five batters in 2018.

When Castillo gets to two strikes, he’s going for the K with the changeup with increasing frequency…

…And he might as well hand the batters a shovel. Check out the progression of his locations for the two-strike pitch via heat maps for his three seasons:

“It looks like a strike but the bottom just falls out of it,” said Giants analyst and former major league pitcher Shawn Estes of a Castillo two-strike changeup that induced the admittedly free-swinging Pablo Sandoval to chase on Sunday (I’ll add the GIF when Baseball Savant slices and dices it). “You have to swing at that, you don’t know if it’s going to be a strike if you do take it. It’s just got so much movement to it, and he sells it with the arm speed.”

The other major point to make is that Castillo’s overall groundball rate (57.8%) is back up around where it was in 2017 (58.8%) as opposed to 2018 (45.9%). Combine that with his strikeout rate and you’ve got a pitcher who stands out in a crowd. Here’s a scatter plot for 2017 and ’18, using 80-inning minimums so as to capture Castillo’s rookie season.

The two red dots represent Castillo; for 2018, all you can really see is a crescent, the point being that he was nothing special with regards to his combination of grounders and strikeouts, particularly when compared to his debut season (that stray dot on the far right is Josh Hader’s 2018, if you’re wondering). Here’s 2019:

A pitcher who misses bats and keeps the ball on the ground is a special creature. Here’s a little junk stat I whipped up using our new “Plus Stats,” specifically league-indexed strikeout and groundball rates, where 100 is league average, 120 represents a performance 20% above average, and so on. I’m adding the two indexed rates together for something I’m calling KGB+:

Of the top 28 seasons in this junk stat, dating back to 2009, six of them were Cy Young-winning campaigns, and another five came from pitchers who won the award at some point. Dominant seasons from guys like Corbin, Fernandez (RIP), Harvey, Strasburg, and Syndergaard are represented as well. Not everybody here was equally as adept at preventing runs, but this is the cool kids’ club, and the way Castillo is pitching, he’s working towards his membership card. That’s not to say he’s a finished product — his to-do list includes trimming his 9.7% walk rate and getting more swings and misses with his slider — but his progress is something to behold.