Ex-Tiger Trevor Rosenthal appears to be back on track with Royals

This much cannot be questioned: Kansas City reliever Trevor Rosenthal is having a better start to this season than last.

It can be hard to remember now — especially with the early success that Rosenthal has had as a Royal — but it was only last year when Rosenthal debuted for the Washington Nationals with these first 10 results: single, single, walk, single, single, walk, walk, hit by pitch, walk, walk.

The stretch was not only labeled at the time as one of the worst in major league history … it also was the first time ever that a pitcher needed five appearances in a season to record his first out.

It’s good, then, to appreciate just how Rosenthal, who also pitched 10 games for the Tigers last season, has come in the course of a year. Here are his results so far with the Royals this season: flyout, flyout, strikeout, single, flyout, double-play groundout.

The underlying results are even more encouraging.

Among pitchers with at least five batters faced, Rosenthal ranks eighth in Statcast’s all-encompassing “expected weighted on-base average” stat. In essence, even when hitters are making contact against Rosenthal this season, they’re not doing so in a forceful way.

So what’s led to Rosenthal’s transformation thus far?

We can get some hints from earlier stories by Star beat writer Lynn Worthy. Here’s one from March 4, where Royals manager Mike Matheny spoke about Rosenthal’s early success in spring training.

“I think he just got back to what he knows works for him,” Matheny said. “Everything just looked timed-up right. I think he was allowing himself biomechanically to have that range to stay square to the plate. He was falling off a lot last year. I think a lot of it had to do with, maybe, a limited range (after Tommy John surgery). To me he looks extremely similar to what I’ve seen him when he’s been at his best.”

We’ll come back to Matheny’s point in a second, but it’s also important to note what Rosenthal said about being off in 2019:

“My slider was a lot better than my fastball and my changeup, which is not normal,” Rosenthal said. “So those were some red flags that I saw early on.”

Rosenthal hits on something important there, and also the pitch that’s been the biggest change from 2019 to 2020: his fastball.

Last year — while having issues locating the pitch — Rosenthal’s expected weighted on-base average against with his heater was .489. In a small sample this season, that number’s down to .077.

So what has changed?

Mostly the way he’s throwing … and we can get a good look at that comparing his pitches to one specific player both this year and last.

I found examples of Rosenthal throwing high fastballs from both 2019 and 2020 to Cleveland’s Jose Ramirez. Rosenthal has transformed his throwing motion, as his arm angle is much more straight up and down compared to last season.

Rosenthal’s release point, according to the metrics, has shifted eight inches horizontally this year toward his glove side, while he’s also releasing pitches about 1½ inches higher than he did in 2019. This also appears to be what Matheny was referring to when he discussed Rosenthal staying “square to the plate” as opposed to “falling off.”

“I think it’s just the normal process after an injury of getting back to being yourself,” Matheny said Wednesday, “and right now, this just looks like Trevor to me, and it looks really good.”

The new arm angle — so far — has done wonders for Rosenthal’s fastball. Last year, Baseball Savant had Rosenthal’s fastball rising 7% more than MLB average; basically, it was coming in mostly straight.

The new mechanics have altered that. Rosenthal’s fastball has added nearly two more inches of vertical life this year, with a break that is now 19% above average.

That — along with his improved command — has appeared to make a big difference.

More evidence came Tuesday night, as Rosenthal zipped a 1-1 fastball by Detroit’s Jeimer Candelario.

Candelario swings under it, with the fastball’s late movement turning potential contact into a whiff.

When the Royals signed Rosenthal to a $2 million contract in the offseason, it definitely could be seen as a gamble. Rosenthal finished with a 13.50 ERA last season (including a 7.00 ERA in nine innings with the Tigers), and there was no guarantee he’d be able to return to his previously dominant form.

It’s early, but it sure looks like the Royals did well for themselves with this deal. Whether Rosenthal continues as a high-leverage stopper or is traded to a contender for value, he appears on track to regain his status as dominant reliever.

He has both an improved arm angle — and a livelier fastball — to thank for that.