The Possible Star of the Pirates Bullpen

If the Pirates are going to make it back into the playoffs, it stands to reason they could need a hell of a bullpen. It’s something they’ve leveraged before — between 2013 and 2015, when the Pirates made the playoffs in all three years, they had the highest bullpen WPA in baseball. It was one of their various subtle strengths, and it made them tougher than many predicted.

Of course, the Pirates no longer have one of the major pieces that lifted them up. Mark Melancon is on the Giants now, by way of the Nationals, who received him last July in exchange for two players. It was almost inevitable the Pirates would sell him, and many compared the move unfavorably to the haul the Yankees got for Aroldis Chapman. Yet, for one thing, look — Mark Melancon isn’t Aroldis Chapman. Don’t be ridiculous. And also, don’t sell Felipe Rivero short. Taylor Hearn is a moderately interesting prospect, but Rivero is interesting as a relief pitcher now, and he might be primed to be the Pirates’ next big thing.

There’s really not a lot I should have to do to sell you. Rivero is 25, and over 125.1 big-league innings, he’s managed an FIP- of 79. He’s a lefty with three pitches whose fastball has averaged almost 96 miles per hour, and his changeup recently came on as a strength. For that reason, last year, among lefty relievers who faced at least 100 righties, his K-BB% against those righties ranked behind only Andrew Miller, Chapman, and Zach Britton. Rivero’s other pitch is a slider, and — the other day, I wrote a little about Brad Hand. I observed that Hand throws a slider that looks a lot like Miller’s. So does Rivero. By PITCHf/x numbers, the pitches are brothers, if short of being twins. Rivero’s repertoire is unusually interesting.

He throws hard, and he mixes pitches. Here is some more data to encourage you. Here is last year’s zone rate plotted against contact rate on pitches in said zone.

It’s not that Rivero is an outlier. There were other, better relievers. But he threw an above-average rate of his pitches in the zone, and he also minimized contact within the zone, relatively speaking. Those are two wonderful things to pair together. Last year, there were 39 relievers who threw at least 50 innings, with at least half their pitches in the zone. Out of that group, the only pitchers with lower in-zone contact rates were Chapman, Edwin Diaz, and Mychal Givens. Rivero allowed essentially the same in-zone contact rate as Kenley Jansen and Kelvin Herrera. That speaks to his ability to completely dominate certain at-bats.

One problem that held Rivero back was that he issued too many walks. You can eyeball the numbers and guess at the perception — throws hard, is wild. Rivero ranked in just the 20th percentile in overall walk rate, and that’s lousy. But then, there’s the fact that Rivero threw so many pitches in the zone. That doesn’t seem like something a wild pitcher would do. And you know what works against walks? Strikes. Interestingly, by strike rate — a very simple stat! — Rivero ranked in the 70th percentile. He threw more strikes than the walks would suggest, and that hints at an improvement.

There’s still more. Rivero didn’t just post a weird walk rate, given his strikes. He also probably should’ve had more strikes. For example, of Rivero’s pitches that were taken in the zone, 20% of those were called balls, against a league average of 14%. I recognize this is getting into the weeds, but Rivero pitched to a pitcher-unfriendly zone last year. He didn’t get the benefit of the doubt in the zone, nor did he get it outside of the zone. Some of that, sure, is probably Rivero’s own fault. But when I adjust his numbers, I end up with his having deserved strikes on two-thirds of his pitches.

Let me make that all simpler: I don’t think Rivero is as wild as he seems. He’s usually around the zone with all of his pitches. That’s intriguing, and it’s no less intriguing to look at how his mechanics evolved as last season wore on. Here’s Rivero from an earlier appearance:

Here he is from much later in the year:

Pay attention to the follow-throughs. The follow-throughs don’t lie, and in the first clip, Rivero’s momentum takes him toward first base. In the later clip, Rivero’s momentum is mostly toward the catcher, with a little leg swing toward third. Rivero, like many hard-throwing young relievers, doesn’t always perfectly repeat his mechanics, but the second delivery simply looks cleaner. Here’s an arrangement of screenshots. Up top, Rivero with the Nationals early, and on the bottom, Rivero with the Pirates late. Look at his body position.

There’s an undeniable correction that was made. From what I can tell, it pre-dated the trade, so we can’t just point at Ray Searage, but he did nothing to discourage this adjustment. Rivero used to fall toward first to find his balance. Look at the angle in his right, landing leg. On the bottom, his leg is almost perfectly straight up. This is, conveniently, a part of the Searage philosophy: Simplify, and put as much as you can toward home plate. The Rivero from the second half had the cleaner-looking throwing motion, and I think it’s only a matter of time before that translates into even better results.

In a sense, it kind of already has. Rivero’s average fastball with the Pirates was faster than his average Nationals fastball by a tick and a half. His June fastball velocity ranked in the 84th percentile; his September fastball velocity ranked in the 97th. Rivero became more efficient with his energy, and thanks to Baseball Savant, we can also see that Rivero gained an additional half-tick in perceived speed. That is, as Rivero’s delivery improved, he added a few inches of forward extension, making his pitches look that much faster. That’s not bad for someone making changes on the fly.

By the numbers, the Pirates have an interesting young southpaw with a bit of a walk problem. By profile, the Pirates have a hard-throwing young southpaw with three pitches and surprising control. His delivery has gotten better and he has the stuff to handle hitters on either side. Even last year, he kept righties in check, which isn’t easy for a lefty to do. Rivero does need to keep working, and he can’t let himself get over-excited. Over-throwing isn’t good for anyone. Yet there is ever so much to like. Rivero strikes me as being the real deal, and that Melancon trade should work out just ducky.