The Nationals' bullpen, in need of support, would be greatly aided by a midseason turnaround by Trevor Rosenthal. (Toni L. Sandys/The Washington Post)

HARRISBURG, Pa. — On Day 25 of a rehab assignment that most players probably wouldn’t have gone on at all, Trevor Rosenthal was here, about 120 miles away from his major league team, pitching for the Class AA Harrisburg Senators in a half-full stadium on a Tuesday night in June, still trying to fix what was wrong.

In the sixth inning, Senators Manager Matt LeCroy summoned the most high-profile rehabbing reliever he has ever had to face the Nos. 6, 7 and 8 hitters in the Binghamton Rumble Ponies’ lineup. The entrance might have seemed low pressure, but it was not.

The Washington Nationals needed Rosenthal to figure it out. They had guaranteed him $7 million this season, a bet that a pitcher who had missed the past 18 months after Tommy John surgery would become their setup man. Quickly, though, they discovered the right-hander had lost his dominance. The procession of hits, plunked batsmen and wild pitches sent him from the majors to Harrisburg, where in his first 7⅓ innings he had walked seven, allowed six hits, hit three batters and surrendered four runs.

After a Binghamton bunt single on his first pitch, Rosenthal got ahead 0-2 on the next three hitters, attacking the strike zone with the consistency that, four days later, would lead the Nationals to bring him back to the active roster a day before his assignment ended. That night, Rosenthal stopped the trouble as it began, retiring those three hitters with a flyout, strikeout and groundout.

“I feel like there’s been a big change in my consistency in … fastball command,” he said. “[I] just got to go back to it.”

The simple adjustment was significant.

“When he has [fastball command], he has pretty easy work,” LeCroy said. “His velocity is still there … but it’s about him getting up here” — he pointed to his temple — “to trust it.”

Rosenthal’s explanation for the progress was simple because when he thinks about why his season has gone so wrong he envisions the reasons in a pie chart. He has thought a lot since he last pitched in the majors April 24.

The chart, he said, looks something like this: The biggest part is what he called “a bad mechanical tendency” to open his front side too soon during his delivery. He believes this could have developed, at least in part, by his body subconsciously compensating for weakness after surgery. Whatever it was, the result has been yanked pitches and degraded fastball command.

Some reasons are not mechanical, Rosenthal said, including “being on a new team and trying to really prove myself.” There also was self-applied pressure to live up to his contract and stress after allowing the bad outings to snowball.

Now, he believes the antidote to all of these issues is pitching “toned back,” to prioritize fluidity in his delivery over trying to throw hard. The 29-year-old believes staying in control is key because, as the frustration grew and the pie chart got crowded, he found himself trying to throw faster and faster, attempting to locate finer and finer. He watched his efforts turn into runs.

While examining those outings and wondering what he could do better, Rosenthal realized that every time he has felt good about throwing — in long toss, during his throwing program, in the bullpen for Nationals brass in Cincinnati — he has felt slowed down.

“My natural instinct to try to get the result [I want] is to try harder, and that’s actually working against me,” he said. “It’s a reality, but it’s also counterintuitive as a competitor. You usually correlate effort with results, and so to take that step back can be hard, especially when you’re in the heat of competition.”

He understands the challenge of this balance, calling it “a tricky algorithm” because it means not tapping into the energy he once needed as a closer to throw his fastball past someone.

“Sometimes everything syncs up and I can add a little bit to [my effort] and it’s fine,” he said. “But right now, my stuff’s good, too. I still have above-average stuff when I’m throwing nice and easy. I think it’s realizing that, having confidence in that.”

Harrisburg pitching coach Michael Tejera found comfort in the reasoning. He said it’s more difficult to rejuvenate a pitcher’s stuff, especially after Tommy John, than his command. The Nationals believe Rosenthal’s stuff is still there because his slider has “looked good,” Tejera said, and his fastball still can hit 100 mph.

The last steps for Rosenthal are smoothing his delivery and honing his release point, said Tejera, who has been through Tommy John himself. Once those parts of Rosenthal’s delivery became slightly more consistent, Tejera said, he found the strike zone more often.

“He’s trying to get back to where he was before,” Tejera said. “[The mechanical fixes are] something that all of a sudden you feel it” — he snapped his fingers — “and there it goes. We’re just waiting for that click.”

On Monday night in the Nationals’ 12-1 win at the Chicago White Sox, Rosenthal made his first appearance since he returned from the injured list. He entered for the ninth inning with that 11-run lead and held it, walking the first batter on four pitches but then getting a grounder for a double play and a flyball to right field that ended it.

Despite this progress, Rosenthal understands better than anyone the fragility of mechanical adjustments. He has tried to actualize the lessons in appearance after appearance, only to get overwhelmed by the moment.

Yet there have been glimmers in Rosenthal’s struggles, like this past Tuesday. For his second out, Rosenthal attacked Binghamton’s Michael Paez with low fastballs. Then he tried a breaking ball even lower, out of the zone, but the second baseman didn’t chase. Then Rosenthal wasted no time. He watched from the mound as Paez swung through a high, on-target fastball. And, in that moment, Rosenthal looked like the all-star closer he had once been. The batter walked, head down, back to the dugout. The pitcher tugged at the brim of his cap.

“It’s there,” Rosenthal said. “I just need to stay right there. Overdoing it is a place I can’t really go right now. I don’t feel like I’m different or anything’s changed. It’s just repeating something I’ve been doing, now just at game speed.”

Read more on the Washington Nationals:

Nationals make MLB history with four consecutive home runs in win over Padres

Nationals select junior college right-hander Jackson Rutledge with No. 17 pick in MLB draft

Svrluga: Even in a rocky outing, Stephen Strasburg shows why Nationals’ strength is their starters