Finding the Next Edwin Díaz

This is Jake Mailhot’s fourth post as part of his May Residency at FanGraphs. A lifelong Mariners fan, Jake now lives in Bellingham, Washington, just a little too far away from Seattle to make it to games regularly, which is sometimes for the best. He is a staff editor at Mariners blog Lookout Landing. He can be found on Twitter at @jakemailhot. Read the works of all our residents here.

Among the various career arcs in professional baseball, the conversion from starting pitcher to reliever is one of the more common ones. It’s a last resort for aging veterans and a tried-and-true way to get the most out of middling starters. But when a talented prospect is moved to the bullpen, there are bound to be questions. It has been generally understood that a starting pitcher is more valuable than a relief pitcher, so teams are usually more conservative with their prospects, often letting them at least try to work things out as a starter before pulling the plug. But in an era when relievers are throwing more innings than ever before, a high-octane reliever might prove to be more valuable than just another starter.

Back in 2016, the Mariners moved one of their best pitching prospects from the rotation to the bullpen. Edwin Díaz took to the conversion quickly and was in the majors a few weeks later, completely skipping Triple-A. He was soon installed as the Mariners closer and has been one of the best relievers in the majors since. His already excellent fastball velocity received the usual boost from shorter stints on the mound, and his slider has developed into a plus-plus pitch.

It was a risky move for the Mariners. Instead of letting the 22-year-old try to develop his changeup in the rotation, they shifted him to the bullpen and aggressively promoted him because the major-league team needed bullpen help desperately.

I wondered if any other teams had tried something similar. Below you’ll see the results of a very specific query: every relief pitcher who has thrown at least 10 innings in the majors and had been a starting pitcher in the minors as recently as last year. To narrow the field even further, these pitchers all recorded fewer than five innings pitched in Triple-A and have posted an average leverage index greater than 1.25 when entering the game.

It’s an interesting list. Jordan Hicks, the man with the fastest fastball in all the land, sits atop it with almost 27 innings pitched and just 16 strikeouts to his name. Then we have a Rule 5 pick, Brad Keller, who has recently been in the mix for high-leverage innings in the Royals bullpen. Moving on. Justin Anderson wasn’t a highly regarded pitching prospect in the Angels organization, but he has added more than 6 mph to his average fastball velocity out of the pen and given Mike Scioscia another option in his constant closer carousel. This article was almost about Anderson. But the final name on the list is far more intriguing — and not just because of his 80-grade baseball name.

Seranthony Domínguez the prospect wasn’t as highly regarded as Edwin Díaz was back in 2016. Eric Longenhagen ranked him 18th in the Phillies organization in 2017 before moving him up to 11th this year. He had some trouble developing his other secondary pitches — a changeup and a curveball — so the Phillies decided to convert him to reliever this offseason. Like the Mariners with Díaz, the Phillies felt some urgency, hoping to see Domínguez move quickly through the system to make an impact at the major-league level sooner rather than later.

Here’s what Joe Jordan, the Phillies’ director of player development had to say about this transition:

“The reason why we took him out of the rotation and put him in the bullpen is we felt like this could happen pretty quickly. We didn’t put May 7 as the date that would be the day. He’s obviously done a lot of good things and that’s why we’ve done what we’ve done.”

And here’s what Gabe Kapler, his manager, sees in Seranthony:

“I think we have something special in Seranthony. He’s a strike thrower and that’s a really good profile for a reliever who throws 97 to 99. Plus he can land his secondary pitch for a strike, and by the way, that’s kind of nasty, too. His fastball has cut and sometimes sinks.”

He was called up on May 7th and has already been inserted into some very high-leverage situations. He earned his first hold in his third appearance, and his first save — a two-inning save at that — in his sixth. He’s has yet to issue a walk or allow a run, and didn’t allow a baserunner until his seventh appearance, a major-league record. The only hits against him have been two harmless singles.

Like Díaz, Seranthony generates huge whiff rates off both his pitches. Among the 398 pitchers who have thrown a four-seam fastball at least 50 times this season, his fastball whiff-per-swing rate of 33.3% ranks 22nd. His slider has been even more impressive. Among the 329 pitchers who have thrown a slider at least 25 times, his slider whiff-per-swing rate of 64.3% ranks seventh.

Here are a couple of big-league batters looking foolish against a couple of Domínguez’s pitches. The first is Chase Utley hopelessly waving at a fastball:

And here’s Steve Baron flailing at a slider:

It’s actually pretty uncanny how similar Domínguez’s mechanics look to Díaz’s. The body types are completely different, but there’s the same violent whipping action from a three-quarters arm slot creating some deception for opposing batters. Domínguez is a cherry-picked comparison for Díaz, but there are enough parallels between the two as to make the narrative arc of their careers interesting.

With the Phillies’ bullpen in disarray after the struggles of their deposed closer Hector Neris, I wouldn’t be surprised to see Seranthony take hold of the ninth inning sometime this year. He has the skill and repertoire. He’s gaining the experience. All he needs is the right opportunity. It took Díaz about two months before he supplanted Steve Cishek in 2016. Unfortunately, Domínguez’s manager might have other opportunities in mind for now. “One of the things that’s nice about him is that he has a history of starting games and he can give us a little more length,” said Kapler after the right-hander’s two-inning save. The Phillies’ bullpen suffered another meltdown Monday night against the Dodgers, a game in which Domínguez had pitched 1.1 innings and struck out three. Maybe he’ll find his way closer to the ninth sooner rather than later.