A bad first period, seven forwards without a shot on goal and a power play that went 0-for-4 added up to a 4-1 loss against the Ottawa Senators at Canadian Tire Centre on Thursday.
The loss ended the Canadiens' six-game winning streak on the road.
The Senators (18-17-8) won for the second time in six games (2-3-1).
"We had a rough start. That was probably our worst first period of the season," Therrien said. "You've got to give a lot of credit to the Senators. They came in hard. We advised our players what to expect. They didn't compete at all in the first period and it ended up costing us the game. We were better in the second and third but it was definitely too late."
Montreal (27-13-3) gave up a goal to Ottawa's Mika Zibanejad 69 seconds into the game. Zibanejad jumped on a rebound in front of goaltender Dustin Tokarski, starting his eighth game of the season. With the Canadiens playing back-to-back games, Therrien opted to rest No. 1 goaltender Carey Price.
Max Pacioretty tied the game at 14:35 of the first period with his 21st goal and extended his goal streak to six games, tying Denis Savard for the longest goal-scoring streak by a Canadiens player since 1991.
But another rebound goal by Jean-Gabriel Pageau at 16:26 of the first period put the Senators in front 2-1.
The Canadiens have allowed the first goal in 25 of their 43 games. Montreal has been outscored 33-17 in the first period this season.
"We came out flat, and once we started to play our game we had a chance to get back into it but we couldn't find a way to solve the goalie," Pacioretty said. "A lot of the game was on the perimeter. A game like that, back-to-back, it's got to be a playoff mentality and we've got to find a way to get pucks to the net and bodies to the net, and that's what you need to beat a hot goaltender."
Ottawa goaltender Craig Anderson made 25 saves in his 12th win, and defenseman Erik Karlsson scored his first goal in 15 games in the second period. Erik Condra scored into an empty net for Ottawa, and Milan Michalek had two assists.
The Senators were coming off a nine-day, 1-2-1 road trip. Ottawa coach Dave Cameron gave his players the day off Wednesday.
"It felt like we had more energy coming out from the start here and we never let off the gas," Karlsson said. "I think for the first time in a while we played three periods really well. It gave us the win."
Anderson said if the Senators are going to keep their dwindling Stanley Cup Playoff chances alive, they are going to have to take advantage of the schedule.
The Senators, eight points out of the last wild-card spot in the Eastern Conference, started a stretch when they will play 12 of 16 games here.
"Take care of business at home. I think the teams that are in the playoffs right now have got really good home records," Anderson said. "For us, take care of business at home, win some games and put some pressure on the teams in front of us.
"I think we had the jump. Montreal traveled last night, they played, so the second night of back-to-back and I think we had the fresher legs. We were able to capitalize, play hard and make it hard for them to attack our net. We really didn't give them a whole lot."
With the Senators up 3-1 after two periods, Anderson made a good save on Brendan Gallagher, who broke behind Ottawa defenseman Jared Cowen in the first minute of the third period. Anderson got his right toe on Gallagher's shot.
Karlsson scored his first goal since Dec. 7 at 2:16 of the second period. He took a pass in the slot from Michalek and his shot beat Tokarski over the right shoulder to give the Senators a 3-1 lead.
The rest of the second period was a steady stream of penalties. There were five minor penalties called over the last 14 minutes, four of them on the Senators, but the Canadiens couldn't take advantage.
The Canadiens power play unit ended a 0-for-16 streak Wednesday, scoring each goal on the man-advantage in a 3-2 win against the Columbus Blue Jackets. It created some chances against the Senators, but Anderson was sharp. His best stop came on the Canadiens' first power play of the second period when he moved to his right to block a shot by Montreal defenseman Nathan Beaulieu from the left-wing circle.
"You can't be perfect every night. We had some good movement on the power play today, we had a couple of looks that at the end of the day we probably just got to shoot and keep it simple," Pacioretty said. "It's easy to judge the power play on the pucks that go in the net, but tonight we didn't lose any momentum on the power play. It wasn't like that. This was a little different story."
Therrien pointed out only the line of Pacioretty, Gallagher and Tomas Plekanec is providing offense for the Canadiens lately.
"We need contribution from more than one line. We've got seven forwards who didn't get a shot on net. This is unacceptable," the coach said.