Frustration had been mounting for more than a year, defeat after defeat and punishing tackles taking a toll on United’s 5-foot-3 central midfielder.

Acosta had been overburdened since arriving 2 ½ years earlier at age 21 and, amid the desolation, there were occasional tiffs with Coach Ben Olsen and tempestuous episodes with opposing players and referees.

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And then this setback …

Acosta fumed. He declined to accompany the team on a tour of Audi Field, the soon-to-open stadium in Southwest Washington. (He stayed home and watched Argentina’s World Cup match.)

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“I was really upset,” he said through an interpreter last week. “I was hurt by what was happening. There were decisions made I didn’t agree with, and I shared my thoughts about how things should happen.”

Acosta met with Olsen and club executives. Among the topics, sources said, was whether he should depart during the summer transfer window. Acosta said he never threatened to leave and that the sides had a good conversation.

United co-chairman Jason Levien, who was in the meetings, said, “He just wants to win so badly.”

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The sides talked it out. United’s message, Acosta said, was to “stay calm and keep on your path. They said things were going to change. They were going to sign players. They didn’t say who. But they kept their promise.”

Two weeks later, United inked former Manchester United striker Wayne Rooney to the largest contract in club history.

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Acosta’s career — and mood — haven’t been the same since.

The partnership — Lucho and Señor Wayne or how about Lucharoo? — is the hottest in MLS. With a seasoned attacker relieving the pressure on him and a partner to create opportunities, Acosta has compiled career-best numbers and played with the joy and expression that the club had envisioned since 2016.

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Before Rooney arrived, Acosta had posted one goal and seven assists in 14 matches.

Since: nine goals and 10 assists in 20 outings.

Rooney has been pretty good himself, with 12 goals and seven assists.

Overall, Acosta’s 17 assists were second in the league and two short of the club record.

The previous two seasons, he totaled eight goals and 16 assists in 62 appearances.

With Acosta and Rooney leading the way, United (14-11-9) is unbeaten in 10 straight heading into Thursday’s first-round playoff match against the visiting Columbus Crew (14-11-9).

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Of his collaboration with the new forward, Acosta said: “It’s Rooney. He’s one of the best. He does everything so well and makes it look easy on the field. It wasn’t hard for me to complement him. You learn his movements and learn how he plays, where he wants the ball and where he is going to serve the ball. For me, it was not difficult to learn.”

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Acosta, 24, was the beneficiary of the season’s most iconic moment: Rooney, the captain, making a game-saving tackle at midfield in the dying moments against Orlando and subsequently serving a long ball to Acosta for the winning header and his first hat trick.

On Sept. 29, they combined for three goals and four assists in a 5-0 rout of Montreal. Three weeks later, Acosta beat five New York City FC players to set up Rooney’s early goal and, soon after, struck from distance.

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With quick feet and a quick mind, Acosta had spellbinding moments in his first 2 ½ seasons. But that’s all they were — moments. His contributions were infrequent, and his defense and work rate weren’t up to par.

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“This season has changed him in terms of his mentality,” midfielder Paul Arriola said. “He has always been a good team player, but now he is playing with more confidence and commitment in what we’re trying to do.”

United was to blame, in part, for relying too heavily on him for Acosta’s first 2 1/2 years.

“We’ve always said behind the curtain we’re asking too much of this kid,” Olsen said. “Before Wayne and the bolstering of this roster, it was up to him. If he was playing at a high level, we were playing at a high level. If he wasn’t, we tended to lose those games.”

Acosta concurred.

“There was a lot of pressure and responsibility that maybe we [Acosta and then-captain Steve Birnbaum] weren’t ready for, or it was just too much," Acosta said. “When Wayne came in, not only did he take that pressure and burden on himself, he helped us understand our roles and divide the responsibility.”

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Acosta has also benefited from a formation change in the summer: two defensive midfielders providing support instead of one. With Russell Canouse and Junior Moreno behind him and Rooney in front of him, Acosta has thrived.

“It was a big improvement for me, but the whole team improved,” Acosta said. “Without them, I wouldn’t be at the level I am playing at now.”

With a renaissance season and Boca Juniors pedigree, Acosta has surely attracted attention from clubs overseas. His MLS contract runs through 2019, at a base salary of $550,000, fourth highest on the roster. To secure his long-term services — or to ensure a transfer fee, if a big offer comes along next year, instead of losing him as a free agent — United may very well open negotiations this winter.

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Acosta said: “I am happy here. I want to be here. This is my focus. I told my agent, if there is anything out there, don’t let me know. You manage it. I want to be able to focus on these games.”

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The happiness factor shouldn’t be underestimated. He likes living in Washington, he is learning English, and he and his wife are raising two young children in Alexandria.

And after a prolonged rough patch, he is winning.

Asked if this is the happiest he has been with United, Acosta said: “I am most comfortable now and feeling great about being here. I’ve been able to connect with the players and enjoying the way we play.