New York coach Mike Petke and match captain Dax McCarty rued the mistake which allowed Marco Di Vaio to score for Montreal.
"(The Impact) are 4-0 now. They have quality players, they're a very organized team," Petke told reporters after the match. "I thought for a large part of the game that we were in control. I think we played in their half a lot.
"But, there's a reason why they're 4-0. They're a good team. And I think we fell asleep a couple times, and one of the times they made us pay. We have four in the back versus one, and he gets through.
"That's something I have to go back and look at and figure out."
Red Bulls midfielder Dax McCarty, who captained the side in the absence of Thierry Henry, echoed his coach's sentiments.
"I thought we played pretty well," the 25-year-old said following the match. "Just a mental mistake, one breakdown in the back.
"Give Di Vaio one chance and he puts it away."