The Montreal Impact paid the price for making some risky lineup changes to start the second half of the season.

Impact rookie defender Calum Mallace and goalkeeper Evan Bush were both victimized in their Major Soccer League debuts as Montreal lost 3-0 to D.C. United on Saturday.

"In some ways we felt like the timing was right," said coach Jesse Marsch. "Because of the miles we had on the different guys' legs and giving certain guys a chance because we felt like they'd have a fresh mentality.

Montreal has been ravaged by injuries and was also playing without suspended midfielder Patrice Bernier.

"Yeah, it's a tough game," said Marsch. "But you've got to put guys on the field and see how they do."

Mallace, the first pick in the second round of the 2012 MLS SuperDraft, was welcomed to the league by D.C.'s leading scorer, Chris Pontius, on the stroke of halftime.

Receiving the ball 35 yards out, Pontius raced toward goal and turned Mallace around with a trademark cut to his right foot and surprising and deadly touch back to his left.

Wide open, Pontius slammed the ball inside the far post and past Bush, who was pressed into duty while Donovan Ricketts was on the bench for the first time all season.

Mallace, who took blame for the first goal, also nearly gifted D.C. in the 25th minute, turning a pass from Canadian Dwayne De Rosario into his own net. Luckily, the ball deflected off the post, and Mallace was replaced at halftime by Jeb Brovsky.

"Sometimes you're on your game, and sometimes you're not," said Mallace. "Personally, I felt like I wasn't on it. Maybe it was a little pressure, maybe not. Either way, I look at myself and feel like I let myself down and the team."

In the stifling 33 C heat, Pontius' goal put the Impact (5-10-3) on their heels. When Robbie Russell connected on a diving header off Branko Boskovic's free kick in the 50th minute, Montreal was effectively knocked out.

Marsch disputed the call on Collen Warner that led to the free kick.

"I don't think it's a foul," Marsch said. "I thought the ref was terrible. This was one of the most one-sided reffed games I've seen. We look at ourselves any time, but the ref did us no favours."

Bush made his fourth save in second half stoppage time but gave up a rebound on the shot to Hamdi Salihi, who knocked in the third goal for D.C. (10-5-3), which moved into sole possession of first place in the Eastern Conference.

Sanna Nyassi had Montreal's best and only chance of the game in 11th minute. Breaking free down the middle, his low shot forced a sprawling save from United goalkeeper Bill Hamid, who tipped it out for a corner.