Trevor Rosenthal dialed back his velocity in favor of command and it produced results Thursday. (Katherine Frey/The Washington Post)

Allow Trevor Rosenthal to reintroduce himself.

The Washington Nationals reliever mopped up the ninth inning of a 5-0 loss to the Arizona Diamondbacks on Thursday night, and he looked almost like his vintage self doing it. Gone was the over-thinker who needed five appearances and 48 pitches to record his first out this season. Gone was what he called “a bad mechanical tendency” to open his front side too soon during his delivery. Gone was the out-of-control pitcher who sprayed balls everywhere, including behind left-handed hitters, as recently as two weeks ago during his lengthy rehab with Class AA Harrisburg.

Instead, Rosenthal overwhelmed the Diamondbacks’ Nos. 6, 7 and 8 hitters in a 1-2-3 inning. The right-hander pounded the strike zone (12 of 14 pitches), commanded his slider to keep batters off balance and struck out two. Earlier this week, the 29-year-old described his struggles in a pie chart and, other than pressures from joining a new team and his $7 million contract, he identified the biggest culprit as deprioritizing mechanics while trying to throw the ball hard. He believed the antidote to his issues was pitching “toned back” and focusing on fluidity.

“Not trying to overdo it or getting too high on one pitch and not getting too low on another,” he said on Thursday night, explaining what keyed his resurgent performance. “Whatever happens, happens. Just let it happen and not try to take a hold of everything and make it be absolutely perfect. That’s how I felt.”

The throttle-down was noticeable. In the first at-bat, Rosenthal set up Diamondbacks first baseman Christian Walker with two fastballs over the middle of the plate that were 94 and 95 mph, respectively, down from his usual speed of 98 to 100. It didn’t seem to matter because Rosenthal changed speeds with two sliders low, and the second one fooled Walker so badly that he swung through the pitch, lost his grip on the bat and watched it sail almost beyond the first-base dugout.

“If there was one thing positive [from today], that was very encouraging,” Manager Dave Martinez said of Rosenthal. “Even in his warm-ups, he looked very relaxed. … The ball was coming out really, really nice. Man, he was pumping strikes today and it was great to see.”

It’s a small sample size, but what a Rosenthal resurgence might mean for the Nationals depends on which direction this season goes. It’s possible the Nationals put the pieces together, push for a playoff spot in a weak National League East and rely on Rosenthal to become the trusted setup man he was supposed to be. It’s also possible they remain a significant number of games back as the trade deadline nears and find themselves confronted by the question of whether to buy or sell.

If the Nationals decided to punt on this season, then Rosenthal seems a logical candidate to deal. His value hinges on situations that still haven’t happened yet: Can he sustain this apparent progress, much less in high-leverage situations? If he can, Rosenthal might resurrect a career that weeks ago seemed shot, rescue a Nationals investment that once looked silly and reassert a contender’s commitment to its postseason hopes.

For now, the outing was simply a step in the right direction after Monday, when he returned from the injured list to face the Chicago White Sox for his first appearance in the majors since April 24. He faced the minimum, erasing a walk with a double play.

On Thursday, Rosenthal built his speed back up somewhat — the fastball sat around 96 and 97 for the final two hitters — but still offered concrete evidence of a new approach. He stayed within himself and traded velocity for control when it mattered most. The difference produced results against shortstop Nick Ahmed, whom he struck out on three pitches, and Carson Kelly.

Rosenthal fell behind the Diamondbacks catcher with a high slider but forced him to foul off five straight strikes until he flew out to left field. That at-bat stuck out to Rosenthal because, in the past, he might’ve tried to get too fine and beat Kelly with a perfect pitch. Now, he just wanted to stay in a good count and make him put weak contact in play.

From shortstop, Trea Turner noticed a difference.

“He’s got such good stuff that all you have to do, for the most part, is make sure you are around the [strike] zone so those guys are swinging the bat,” Turner said. “Tonight, he was really good and hopefully he can be big for us the rest of the year.”

Perhaps better than anyone, Rosenthal understands the ground left to cover. In a meaningless final inning of a Nationals loss, he wanted to be efficient and set himself up to bounce back the next day. He acknowledged things might’ve been different if tasked with protecting a one-run lead, when a home run could tie the game.

“It'll take time and situations that'll arise in the future,” he said. “But we know we got a good group of guys and, to get where we want to be, it's going to take every last one of us, and I'm just hoping to be a part of that.”

The Nationals, no matter which way their season heads, hope so too.

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