Alejandro Romero Gamarra scored the lone goal for the New York Red Bulls against D.C. United. (Brad Penner/USA Today Sports)

HARRISON, N.J. — D.C. United‘s playoff fortunes will rest with its success or failure at Audi Field, not with the few away matches left in a final two-month stretch dominated by home dates. By continuing to win at its new venue in the District, United is poised to make up ground and capture one of the last postseason berths.

The short trip north this weekend, however, bore opportunity to sustain momentum, further boost the long-term cause and provide a true measure of this rejuvenated team. And it came against its fiercest rival on national TV.

A promising first half Sunday did not yield a goal, and after the New York Red Bulls went ahead on a fine strike and Wayne Rooney‘s sure thing skimmed the crossbar, United absorbed a 1-0 defeat and the end of a five-game unbeaten streak.

The more important matches come this week in Washington against surging Philadelphia and front-running Atlanta, but for United (7-10-6), at least one point was there for the taking.

“We know if we win our home games we’ll make the playoffs,” defender Steve Birnbaum said. “That’s obviously key to the rest of our season. But we wanted to win this game. It’s a big rivalry game. The guys put in a good shift, but they got the goal.”

Alejandro Romero Gamarra, an Argentine playmaker known as Kaku, scored early in the second half before an announced crowd of 22,324 at Red Bull Arena. The outcome clinched the Atlantic Cup (the annual season series between the teams) and extended New York’s unbeaten run against D.C. to seven.

United, which has not won here since the 2012 playoffs, is six points behind sixth-place Montreal for the last playoff berth in the Eastern Conference. Having played at least two fewer games than the other contenders, D.C. remains well positioned to continue climbing.

Nine of the last 11 matches are at home, with the only remaining trips to New York City FC in two weeks and Chicago in the finale.

“The rest of the season is going to be a challenge,” said Coach Ben Olsen, whose team is 1-9-5 on the road. “We are ready for that part and understand the significance of each game now. We are in that stretch where every point matters, in particular taking care of your home games and getting three points.”

United would have been satisfied with one point on the road. Enjoying a three-game winning streak that was its longest since last August, the visitors performed with supreme confidence in a scoreless first half and generated several quality chances.

Rooney and Yamil Asad tested Luis Robles from distance, and Oniel Fisher squandered a header from inside the six-yard box.

Olsen, though, had to turn to his bench four minutes into the second half when Asad, who had hurt his left ankle in the 40th minute, came off. (He is questionable for Wednesday’s match against Philadelphia.)

The Red Bulls (16-6-4) set the terms in the second half. Bill Hamid came to United’s rescue in the 52nd minute with a superb save on Gamarra’s one-timer from eight yards.

Four minutes later, Gamarra was clinical and merciless.

Bradley Wright-Phillips won a header on Marc Rzatkowski’s cross. Gamarra settled the ball beyond the top of the box, eluded substitute Zoltan Stieber and ripped a 22-yard shot into the far lower corner for his fourth goal of the year.

Rooney’s prime opportunity to tie it came in the 66th minute. After working a combination with Luciano Acosta in tight space, he gathered a deflection at the edge of the six-yard box and smashed a shot that beat Robles and nicked the crossbar.

“I’m disappointed,” he said. “I should have scored it.”

Without prompting, Rooney also complained about referee David Gantar. Moments before the goal, Gantar had awarded a questionable free kick to the Red Bulls near midfield.

“The referee told me just before they scored that it wasn’t a free kick but he needed the time to see” an injured player, Rooney claimed. “Which is why he gave the free kick, which is extremely disappointing in a derby game. It’s not acceptable.”

It did not directly impact the goal, by any means. And by and large, United labored against the proactive Red Bulls in the second half.

“It’s hard to play the way you want to play against them,” Olsen said. “They make you adapt to them. You have to be super focused as they are coming right down your throat, and they are going to come right down your throat again and wear you down. You have to sustain that discipline and focus. They wore us down a little bit in the second half.”

With vital home matches looming — and in need of an energy boost — Olsen pulled his most important attackers, Rooney and Acosta, in the 79th and 80th minutes, respectively. If his team could have taken a point, Olsen would have been thrilled. Instead United came up empty, suffering a shutout defeat to the Red Bulls for the second time in 4 1/2 weeks.

“It hurts to play a rival and not get a result,” midfielder Paul Arriola said, “but now we flip the page, and we have to pick up points.”

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