Kofi Opare, a native of Niagara Falls, Ont., scored the only goal in the Los Angeles Galaxy's 1-0 victory Wednesday night at the StubHub Center.
The shutout extended the Impact's scoreless streak to a team-record 273 consecutive minutes. Montreal (13-12-7) also lost its third successive game and extended its unbeaten streak to six games to match team records in each category.
"We created two or three chances in the first half and we didn't score," Impact forward Marco DiVaio said. "But we don't have time to think about that. Now is the time to try to find a solution, fast."
After amassing a league-best 29 points through June 19, the Impact earned only 17 points since.
The Impact must defeat the Philadelphia Union on Saturday in Montreal's Stade Saputo to remain in playoff contention. Montreal, Philadelphia and the Chicago Fire each have 46 points to share fourth place with two games left.
"It's a six-point game, for sure," said Impact goalkeeper Evan Bush, who made his second career start in place of Troy Perkins. "But anything that gives us a shot to get in the playoffs, we'll take right now. We're still in a good spot."
Opare, in his fourth MLS start, scored in the 68th minute. Laurent Courtois began the scoring sequence by passing to Jose Villareal after securing the rebound of Bush's save against Galaxy defender Leonardo.
Villareal's shot from the edge of the penalty area ricocheted off teammate Gyasi Zardes to Opare, whose six-yard shot deflected off Bush's right arm into the net.
Montreal's best chance came in the 55th minute. Karl Ouimette headed Andrea Pisanu's corner kick to Hassoun Camara, whose bicycle kick found Wandrille Lefevre. But Lefevre headed his four-yard shot over the crossbar.
Lefevre, Ouimette and Bush were among six players in the Impact's lineup who made seven or fewer starts. Perkins, Alessandro Nesta, Justin Mapp and Jeb Brovsky did not make the trip.
"We had a lot of guys, especially on the backline, who haven't played a lot of minutes lately," Bush said. "But I thought we had good character throughout the course of the game. There was a will and a spirit. Hopefully, we can build on the effort and push forward Saturday."
The Galaxy (15-11-6) played without Landon Donovan, who had a sprained left ankle. Donovan needs one goal to set the all-time scoring record for MLS.