Therrien was in a slow boil after the Canadiens lost by a four goals for a second time in a row — a 7-3 loss to the Philadelphia Flyers on Monday night.
Along with a 5-1 defeat in Toronto on Saturday night, that's two in a row since they were assured of reaching the NHL post-season with a win in Buffalo last week.
"Since we clinched, our intensity and concentration level, our attention to detail and our work ethic, have fallen a lot," he said after the loss that helped the struggling Flyers end a four-game losing run. "And you see the results.
"It's a game of pride. You have to come to the rink ready to play. That's what we were not doing the last two games. There's no work ethic there right now. It's disappointing."
The Flyers (18-21-3) matched their season high in goals and kept their slim playoff hopes alive.
Goaltender Carey Price, pulled after letting in three goals on four shots in Toronto, gave up two on his first five shots this time. He was yanked again in favour of Peter Budaj at the end of the second period after allowing six goals on 29 shots.
Scott Hartnell led the barrage with three goals, while Claude Giroux and Jakub Voracek each had a goal and two assists. Wayne Simmonds and Erik Gustafsson also scored.
"Carey Price obviously didn't have his best night," said Hartnell, whose team had mustered only three goals in its previous four games. "We had traffic in front, we were making plays, we were moving our feet in the offensive zone.
"We weren't standing still with the puck. It's lot more fun to play games like that instead of 0-0."
Max Pacioretty, Alex Galchenyuk and Brendan Gallagher scored for Montreal (26-11-5), which is in danger of losing three in a row for the first time this season when it plays at first-place Pittsburgh on Wednesday night.
"We always bounced back all season," said defenceman Francis Bouillon, who was minus-3 for the night. "We've got a lot of character in this room.
"It would be nice to win a few games. We're not focusing on the playoffs now, but it's important for us to play some good hockey before the playoffs."
They are likely to be without forward Ryan White, who was ejected 6:11 into the game while his team was already down 2-0 for a hit to the head of Flyers defenceman Kent Huskins. He left the game with what the Flyers said was a concussion.
White, likely to face further discipline, launched himself at the veteran Huskins, but missed his body and caught him square on the jaw. Already in trouble with Therrien earlier in the season for taking ill-timed penalties, White's future with Montreal now looks in doubt as well.
The Flyers outshot Montreal 33-28 in a sloppy game that had the 21,273 at the Bell Centre booing the home side.
Price had a nightmare start again as he let in two on five shots before settling down as the Flyers outshot Montreal 17-8 in the period.
Simmonds broke down right wing and saw his first shot stopped by Price, but the rebound hit rookie defenceman Nathan Beaulieu and went in at 2:45.
Gustafsson's wrist shot from the point went through several bodies in front and trickled between Price's pads at 5:49.
Defenceman Andrei Markov, struggling since his regular partner Alexei Emelin blew out a knee last week, said Price shouldn't take all the blame.
"It isn't easy to play every game at the same level," he said. "Other players can make a mistake and your partner can cover you. We support him."
Consecutive cross-checking calls on the Flyers' Oliver Lauridsen allowed the Canadiens to tie it at Pacioretty tipped in a Tomas Plekanec pass in the slot at 19:02 and Galagher scored from the front of the net 38 seconds into the second frame.
Only 24 seconds later, Josh Gorges overskated a puck behind his net and Claude Giroux pounced to feed Voracek in front and put the Flyers back in the lead. Voracek's 18th goal tied a career high set last season.
Hartnell scored from the slot on a power play at 7:30, but Galchenyuk fought off a check to score at 9:11 before Giroux put one in off Andrei Markov's stick at 10:46 for a 5-3 lead.
Hartnell snapped a shot past Budaj from the top of the left circle 12:45 into the third.
The Flyers were already missing defencemen Braydon Coburn, Andrej Meszaros and Nicklas Grossman to injuries.
Notes: As at many sports events across North America, a moment of silence was held for victims of the Boston Marathon bombing. . . Montreal rested banged-up forward Brandon Prust and put White back in the lineup.