"The second period was atrocious," Julien said. "It was unacceptable.
"There are no excuses about playing with five defensemen. We have to look in the mirror."
Boston got the first goal at the 17:35 mark. Andrei Markov cleared the puck to the Boston blue line, where Dennis Seidenberg kicked it forward, sending Milan Lucic in on a 2-on-1 break. The big wing slipped a pass to Campbell for a shot into an open side.
Plekanec tied it 9:16 into the second when he beat Rask from a tight angle with a medium-speed wrist shot.
A burst of speed from Brendan Gallagher caused confusion in the Boston defense and Pacioretty backhanded Raphael Diaz's rebound in from the slot at 17:42. Pacioretty's last nine goals have all come in five games on home ice.
"The first one was off to the side of the net and I gave (Plekanec) too much net to shoot at," Rask said. "The second was a high rebound and he scored.
"But if you look at how we played in the second period, you can't be happy with that."
Montreal has beaten Boston four straight times dating to last season.
"I'm proud of my players," said Canadiens coach Michel Therrien, whose team plays again Saturday night at home against Buffalo. "Even if we had a slow start, we adjusted well in the second period.
"My players emptied the tank and they were rewarded."
The teams meet again Jan. 30 in Boston.
NOTES: Montreal dressed Douglas Murray and George Parros, and sat Francis Bouillon and Ryan White, who both played Wednesday in New Jersey. Rene Bourque remains sidelined with an upper-body injury. ... Parros got his first shot on goal in seven games with the Canadiens. ... Adam McQuaid (lower body) was out for Boston, while Jordan Caron and Kevan Miller were healthy scratches. ... As in other NHL rinks, a moment of silence was held for Nelson Mandela.