Jose Goncalves scored the only goal in the 31st minute as the New England Revolution downed the listless Impact 1-0 on Saturday afternoon, extending Montreal's winless run to five games.
The Impact (13-11-7), who were on top of the league in June with a 9-3-2 record, began the day in a playoff position in fourth place, but the race got tighter after losing a third home game in a row.
They have three games remaining in the regular season, including a match Wednesday night in Los Angeles and a must-win game at home next Saturday against Philadelphia.
A victory against the Revolution would have greatly strengthened the Impact's hopes of reaching the post-season in only their second year in MLS, but they came out flat. New England (12-11-9) had the lion's share of scoring chances and allowed Montreal few opportunities.
"I think we're all a little bit shocked," said Montreal captain Davy Arnaud. "Considering how big the game was, something was missing.
"We weren't ourselves. We just didn't have our normal urgency going forward to get chances. We lost some duels we normally win at home. I can't put my finger on it."
Montreal will play in Los Angeles without Arnaud or defender Matteo Ferrari, who each will serve one-game suspensions for picking up their fifth yellow card.
Coach Marco Schallibaum and the players spoke all week of the importance of beating New England, but they weren't in the game from the opening kick-off.
"We weren't good today," said Schallibaum, whose team has one of a possible 15 points from its last five matches. "We didn't get much penetration in the box.
"It was more in the head than in the legs. Now we have to pick ourselves up and get ready for the next game."
Arnaud was at the centre of a grim moment in the 19th minute when New England forward Sene Sene was wheeled off the field on a stretcher and taken to hospital with what the Revolution later said was a dislocated left ankle and broken left fibula.
The less than sellout crowd of 18,917 went silent when Sene went down after going for a ball in the Montreal penalty area. His foot got caught between Arnaud's legs and appeared to snap. It was pointing the wrong way as he lay on the pitch in agony.
"I feel really bad about what happened," said Arnaud. "It looks like it's a really bad injury.
"I think it was a good tackle. I won the ball clean, but his foot might have got caught underneath me. You never want to see that happen. I still feel bad about it. I hope he's OK."
No foul was called as Arnaud got to the ball first.
"You run into situations on the field that are bigger than the game," said Revolution coach Jay Heaps. "Everyone's heart went out to him.
"It's devastating for both teams. But the response from our guys — you just felt there would be a moment when our guys had a chance to make a play. We got that goal almost 10 minutes after that incident, so I was really happy with that."
Juan Agudelo just missed heading in a free kick, but New England got the ball back for Diego Fagundez to slip it across the area for Goncalves to score his second of the season.
The New England defence did the rest as they clogged up the middle of the field and allowed league scoring leader Marco Di Vaio and his Impact teammates few looks at goal.
The win stretched the Revolutions' unbeaten string to four games and kept their playoff hopes alive.
"It was crucial," said Heaps. "We knew these three (points) were the most important because they were pulling away.
"Now it's anyone's ball game here for the last two (playoff) spots. That's all you can ask for."
The Impact got some life after bringing in three substitutes early in the second half but New England still got the best chance on a Chris Tierney free kick that went off the crossbar.
The Impact have a handful of players playing through minor injuries, including Di Vaio, Patrice Bernier and Justin Mapp.
But Arnaud dismissed the notion that the Impact, an older than average team, is wearing down at the end of the season. He feels they just need a spark to feel young again.
"You need a positive result to get you back on track," he said. "When you're so close and you feel you're almost there in the post-season and things aren't going well, you just need a bounce, something to go your way.
"Maybe scrape out a win you don't deserve. Something that can turn things around."
Notes: Alessandro Nesta returned to Montreal's lineup after missing two games with a groin injury and Hernan Bernardello was back after sitting out five contests with a sprained ankle. Sanna Nyassi moved into the starting 11 in place of Felipe Martins. Maxim Tissot replaced injured fullback Jeb Brovsky.