Montreal conceded two goals to the Portland Timbers after halftime en route to a 2-1 defeat at Saputo Stadium on Saturday afternoon.
After a strong first 45 minutes, the Impact were sluggish and uninspired after the restart, conceding goals twice within six minutes.
"It's unacceptable when you're at home," said Evan Bush of the team's second-half performance. "We have to figure out a way to come out a lot stronger. When you don't go up a goal, especially at home, you put yourself in jeopardy.
"It's difficult for us to be trailing in these games."
Defender Nat Borchers scored the first one for Portland (3-3-4) in the 64th minute of play and midfielder Diego Valeri got the winner in the 70th.
Two minutes after coming into the game, Dominic Oduro got one back for Montreal (0-3-2) in the 71st, but the comeback fell short.
"If we pushed them in the second half and were more aggressive, we would have won the game," said Nigel Reo-Coker. "But they were more positive and they played more like a team than we did.
"We haven't had many good second halves yet so far, and we need to sort it out."
Montreal had the bulk of the chances to begin the game, as much of the scoreless first half was spent in Portland's end. Montreal controlled play in midfield and threatened with good runs down the wing, namely by Reo-Coker.
The Impact took five corners in the first half and directed six shots towards Adam Kwarasey's net, though three of those were blocked or deflected by Portland's defenders.
The home team's best chance in the first 45 minutes came off a free kick by Marco Donadel in the 32nd, which Kwarasey punched away.
But Montreal was less convincing after the restart.
In the 56th, Portland midfielder Rodney Wallace beat Bakary Soumare to the ball and fired a left-footed shot on net, forcing a fingertip save by Bush. On the ensuing corner, Dilly Duka had to clear the ball off the goal line.
And it was a poor clearance on a corner kick in the 64th minute that allowed Borchers to get the game's first goal, an easy tap-in header. With the tally, Borchers ended Portland's goalless run at 281 minutes of play.
Six minutes later, Valeri, back from injury and making his first start of the season, curled a beautiful shot out of Bush's reach to put the Timbers up 2-0.
"We need to play better," said head coach Frank Klopas. "We have to push the games and there needs to be more accountability. I need to see a lot more from my players."
After Oduro got one back in the 71st, the Impact looked to have equalized late in the game but Jack McInerney's headed goal was disallowed in the 78th.
After initially calling it a good goal, referee Mark Geiger changed his decision upon checking with his linesman. McInerney pushed a defender just prior to scoring.
"I have to see the tape to see if there was a push or not," said Klopas. "But on their corners, I see a lot of guys get pushed around and I don't see a call."
Down 2-1, Soumare missed a glorious chance to tie the game when his header flew over the crossbar in the 87th minute.
With the loss, Montreal remains last in the Eastern Conference with two points from five games. Saturday's defeat was the Impact's first at Saputo Stadium in 10 combined games in all competitions.
Notes: Defenders Hassoun Camara, Donny Toia and Victor Cabrera were out with minor injuries. … Montreal's last MLS win was in Sept. 2014. They are 0-4-6 since. … The Impact have scored three goals and conceded eight so far this season.