Pitching is weird. Development commonly follows an uneven timeline, with progress being erratic, often unpredictable. One little change can mean the difference between life in Triple-A and 20 million dollars, so if there’s one thing to try to accumulate, it’s youth. Young pitchers come with the benefit of more time. It’s hard to know what they’ll do with it, but at least they have it to begin with. More time to find the adjustments that matter.

The Reds can sometimes be an easy team to forget. Their rebuild, admittedly, remains a work in progress. Yet one thing they’ve certainly done is collect young starting pitchers, which gives them that volatility and upside, even beyond the already volatile Homer Bailey and Anthony DeSclafani. Maybe this year will be the year for Cody Reed. Maybe it’ll be the year for Amir Garrett, or Robert Stephenson. Not to leave out Sal Romano. Not to leave out Brandon Finnegan, or Tyler Mahle. Not to leave out all the other candidates. With a few new pitches, or with a few mechanical tweaks, the Reds could suddenly have something special on their hands.

What the Reds have desperately needed to do is develop quality pitching. There’s plenty more development remaining to be achieved. Among the whole assortment, however, there’s one shining light. There’s not really anything left for Luis Castillo to do. He’s an electrifying starter who already made his final adjustment on the fly.

It’s not as if I’m the first person to give Castillo his due attention. He would’ve been a hard young pitcher to miss. But with a new season right ahead, I feel like it’s important to re-establish just how good Castillo has already become. He’s not even a guy who ought to be flying under the radar, since last year, among all starters, only Noah Syndergaard and Luis Severino threw harder average fastballs. Castillo has a hell of an arm. But the Cincinnati thing presumably works against him. So does his own professional history.

A number of years ago, Castillo was one of two players the Giants traded for Casey McGehee, on purpose. Later on, the Marlins tried to trade Castillo to the Padres in the big Andrew Cashner deal, and after Castillo was returned to the Marlins within days because of the Padres’ playing games with medical records, the Marlins shipped him away again, as part of the package for Dan Straily. Castillo has technically been traded four times, and he’s never been a Baseball America top-100 prospect. In fact, when the Giants first moved Castillo away, he’d been pitching as a reliever.

It was the Marlins who allowed Castillo to start. In 2015, he struck out nearly 20% of his opponents, against low-level competition. In 2016, he again struck out nearly 20% of his opponents, against low-level competition. In 2017, pitching as a member of the Reds, he struck out nearly 27% of his opponents, against some of the toughest competition in the world. Castillo earned a midseason promotion to the bigs, and he didn’t slow down.

Even more, Castillo started to show off his durability, getting increasingly familiar with the starting role. Back in 2016, Castillo faced about 20 batters per start, and he made two starts in which he threw at least 90 pitches. This past year, Castillo faced about 23 batters per start, and he made 19 starts in which he threw at least 90 pitches. Castillo proved he could start, and he proved he could start without wearing down. In the majors, when Castillo faced an order the first time, he allowed a .260 wOBA, with a K-BB% of 16%. When he faced an order the third time, he allowed a .241 wOBA, with a K-BB% of 19%.

Earlier, I mentioned an adjustment Castillo made in the majors on the fly. Already, in the minors, Castillo had worked to improve both his changeup and his slider. That’s what allowed his strikeout rate to surge forward. But after making some starts with the Reds, Castillo unveiled a fourth pitch. The following plot comes from Brooks Baseball. You’re seeing Castillo’s game-to-game pitch usage.

Suddenly, on July 25, Castillo went from throwing one kind of fastball to two. Zach Buchanan wrote at some length about the development on July 30. It all happened quickly, after Castillo toyed around with a sinker on the side. Castillo hadn’t thrown a sinker. Apparently not in a game as a professional. And then, there it was, with Castillo offering a pair of fastballs in the upper 90s. What Castillo doesn’t have is a rising four-seamer. He doesn’t have one of those high-spin Justin Verlander heaters. The sinker, though — the sinker moves, and the two fastballs are distinct. They make each other better, to say nothing of the other two pitches.

Here’s how Castillo threw his fastball in the majors, through July 20. This is from the catcher’s perspective.

Mostly down, mostly arm-side. Compare that to what happened next. Beginning on July 25, here’s how Castillo threw his two different fastball types.

The sinker was mostly lower, arm-side. That freed up the other fastball to do something else. Castillo raised his average four-seamer, and he used it more to attack the glove side. Castillo had different fastballs for different situations, and, even better, the new sinker made for a better pairing with the changeup Castillo already had established.

I completely understand that we’re dealing with some limited samples here, but take a look at how Castillo closed out his season. Beginning on July 25, when Castillo started to use a sinker, here are his percentile rankings among starters in certain relevant categories.

Castillo didn’t become completely unhittable. He didn’t strike batters out like peak, healthy Syndergaard. But he managed to do three things at once: strike out a lot of guys, limit walks, and keep the ball on the ground. That launch-angle bar puts pitchers in ascending order, so what you’re seeing is that Castillo was tremendously difficult to lift. There aren’t really a lot of high-strikeout and high-grounder starters. Charlie Morton just blossomed into one. A pretty good comp here would be Carlos Martinez, except that Castillo probably has a better changeup. He might not struggle so much against lefties.

In the Buchanan article I linked earlier, he writes about a sinker Castillo threw to strike out Miguel Rojas. I’d like to show you a few other pitches from moments earlier in the same game. Here’s Castillo throwing a first-pitch changeup to Ichiro.

Following consecutive balls, Castillo evened the count with a sinker Ichiro thought was going to drift up or inside.

That set up the putaway. Castillo came back with a different fastball, one with six inches less tail, and four inches more rise.

Ninety-eight miles per hour, in the bottom of the eighth. I’m not saying Ichiro struck out specifically because of Castillo’s adjusted fastball mix. It’s just hard to be ready for one pitch in the upper 90s. It’s only harder to be ready for two, two pitches that move in different ways. Castillo’s repertoire is as good as it needs to be.

Say what you will for the Reds’ record of pitcher development, but Castillo has developed. Maybe it’s something the Reds did, or maybe Castillo’s progress was always going to be unstoppable, no matter where he was throwing. An arm like his might be destined to eventually find its level, no matter what. The answer isn’t important to me. Might not even be important to the Reds. What is important to the Reds is that the only thing Luis Castillo has left to do is keep on staying healthy. He already has the arm. He already has all of the pitches. Eight months from now, it might not be very hard to identify the best starting pitcher in the NL Central. Castillo simply has to keep himself together.