"Can't blame Erik, it wasn't intentional," Senators coach Guy Boucher said. "A 5-on-3, and they got life. And then they got another one and that was it. That's how they got the momentum."
Lehkonen gave the Canadiens the lead at 7:54 when he scored on the power play to make it 2-1. It was his first goal in 25 games.
"The first goal gave us a lot of momentum," Lehkonen said. "That power play was big for us. We kind of started going from there and it was a huge thing, for sure."
Lehkonen got his second of the period to make it 3-1 at 18:47. Plekanec got the rebound off Brendan Gallagher's shot from the point and slid a backhand pass to Lehkonen, who scored into an open net.
The assist was Plekanec's 600th NHL point (230 goals, 370 assists).
"I'm not a big stats guy but it's definitely a good feeling," Plekanec said.
Plekanec followed up the milestone by making it 4-1 with his first goal since Dec. 14 at 16:57 of the third.
"We couldn't get any traction going into the third," Karlsson said. "We tried to push for it, but they kept playing the game and made it hard for us. They didn't back up and didn't give us any easy opportunities.