Juan Pablo Angel scored in the 82nd minute to give Chivas USA a 2-1 victory in front of an announced crowd of 10,376 at the Home Depot Center.
Montreal played without forward Bernardo Corradi and defender Matteo Ferrari. Corradi, who leads the team with four goals, suffered a bruised left calf Monday in practice.
Ferrari, a veteran of Italy's Serie A, will be out four-to-six weeks after injuring his right quadriceps muscle. Ferrari played in the Impact's first 14 games, making 13 starts.
Yet Impact coach Jesse Marsch said his players compensated for Corradi's and Ferrari's absence.
"The guys on the field knew what the job was and, for the most part, took care of the job," Marsch said. "For the most part, I thought that game was under control. We played well enough to get a result but we let ourselves down."
Angel, who entered the game in the 77th minute, scored on a 12-yard header off Jorge Villafana's left-wing cross.
Felipe Martins scored for the Impact (4-8-3), now winless in four of its past five matches.
The Impact had a chance to take the lead just over a minute into the game.
Andrew Wenger received Martins' through ball, beat two defenders and took a 15-yard shot toward an open net. But Chivas goalkeeper Dan Kennedy charged forward and blocked Wenger's shot.
Then Chivas (5-7-3) went ahead in the 14th minute on Alejandro Moreno's deflection of Juan Agudelo's shot off Cesar Romero's corner kick.
"We make critical mistakes," Marsch said. "We talked about dealing with set pieces and dealing with crosses, and that's what hurt us."
Martins tied the score in the 42nd minute. After passing to Davy Arnaud on the right flank, Martins redirected Arnaud's cross inside the left post from eight yards.
Montreal had two late chances to tie the score. But Kennedy dived in front of the right post to grab Sanna Nyassi's low 24-yard shot in the 90th minute, and Martins fired his 26-yard free kick just over the crossbar in stoppage time.
"We've had a lot of games that have looked like this," Marsch said. "We feel like we're good enough on the day to get a result, and we wind up somehow giving it away."
Marsch said that such a tendency is hampering the Impact's progress.
"We're working like hell to push this thing along and establish ourselves on a real level," Marsch said. "In a lot of ways, we're there. We've just got to figure out how to manage these games."