The save spoiled United’s quality second half and all but secured a 2-1 victory before 17,372 at Red Bull Arena.
“I should bury it,” said De Rosario, whose 103 regular season goals are sixth on the league charts.
The Red Bulls (12-9-6) also claimed the Atlantic Cup, presented to the winner of the season series, for the third time in four years.
United (3-18-5) is 1-7-2 in the past 10 outings and 0-10-3 with only four goals scored on the road.
“It’s the way it’s been going all year,” midfielder Perry Kitchen said.
United trailed after eight minutes, tied it on Nick DeLeon’s goal late in the 36th, then conceded Tim Cahill’s go-ahead header two minutes later.
United generated several quality chances in the second half but hit the crossbar, missed a penalty kick and couldn’t take advantage of a shorthanded opponent in the last 20 minutes.
“The character we showed . . . I am proud of the performance,” United Coach Ben Olsen said. “I keep saying it: I like this team. I like the future for D.C. United. I am not blowing smoke.”
The Red Bulls smoked United in the early stages. Dejan Jakovic was fortunate not to surrender a penalty kick and receive a red card. His challenge on Cahill appeared to get more of the Australian forward than the ball, but referee Juan Guzman allowed play to continue.
As the Red Bulls howled for a penalty, Lloyd Sam chipped an acutely angled shot with perfect arc and weight. The ball floated over goalkeeper Bill Hamid, struck the back post and crossed the goal line ever so briefly before Hamid could slap it clear.
Nine minutes after Lewis Neal missed a gaping net, DeLeon scored his first of the year. De Rosario crossed to the far side of the penalty area. DeLeon chested it down and, with a nasty cutback, sent defender David Carney tumbling. He then slotted a six-yarder past the advancing Robles.
The joy was short-lived. Unchallenged on the wing, Jonny Steele crossed to Cahill, who beat Daniel Woolard for an eight-yard header beyond Hamid’s reach for his team-leading eighth goal.
United set the terms after halftime. Luis Silva’s header hit the crossbar, and Neal’s blast tested Robles. In the 72nd minute, De Rosario snuck around Ibrahim Sekagya, who was shielding the ball in the box as Robles approached. As De Rosario poked at it, Sekagya brought him down.
Robles, though, foiled the penalty kick. Last fall, he thwarted Chris Pontius’s attempt in the first leg of a playoff series that United ended up winning.
“Even without the PK, we could have maybe gone on and won it,” Neal said. “We played some pretty decent stuff. At the end of the day, though, we still lost.”
United notes: Pontius (hamstring) missed his seventh consecutive match. The club is taking a cautious approach in hopes he will be ready for the U.S. Open Cup final Oct. 1. . . . De Rosario, Jakovic and Kyle Porter will report to Canadian national team training camp in Spain and prepare for a Sept. 8 friendly against Mauritania. They will miss the Chivas USA away match next weekend.
●SPORTING KANSAS CITY 2, RAPIDS 1: Graham Zusi scored in the 88th minute to lift Kansas City to a home victory over Colorado and into a three-way tie for first place in the Eastern Conference.
Benny Feilhaber also scored for Kansas City (12-9-6), which broke a two-match scoreless and losing streak and pulled even with Montreal and New York atop the East.
●IMPACT 0, UNION 0: Troy Perkins made four saves to help Montreal play to a scoreless draw on the road against Philadelphia.
●SOUNDERS 1, CREW 0: Eddie Johnson scored in the 14th minute to lift Seattle to a win over Columbus on the road despite playing down a man for 83 minutes. Johnson gave Seattle (12-8-4) the lead on a header off a 35-yard free kick by Mauro Rosales for his seventh goal.