Boychuk was battling with Pacioretty for a puck along the end boards in the Montreal zone when Pacioretty checked him early in the first period.
Boychuk immediately went down and remained on all fours for several minutes while Boston medical staff attended to him before calling for help from the Montreal trainer, who then called for a stretcher to be brought out.
Boychuk was immobilized before being wheeled off the ice after a 10-minute delay. Pacioretty was assessed a minor penalty for boarding at 4:28.
"Johnny Boychuk has been taken to Montreal General Hospital for observation, after being injured in Thursday's game against the Canadiens," Bruins general manager Peter Chiarelli said in a statement. "Before he was transported to the hospital, he was able to demonstrate movement in all of his extremities. We will provide a further update when it is appropriate."
Bruins coach Claude Julien said after the game Boychuk was returning to Bell Centre and would travel back to Boston with the team.
"But he's not out of the woods yet," Julien said.
Pacioretty was taken off the same ice surface on a stretcher in 2011 after a hit from Bruins defenseman Zdeno Chara.
"I feel terrible. I've been in that situation before," Pacioretty said. "As you can tell from my game right after that, it definitely rattled me a lot. I wish him nothing but the best."
Pacioretty said he hadn't seen the replay immediately after the game, and called it a "battle play" gone wrong. Julien did not disagree with that assessment.
"For me, it was a two-minute penalty and that's it," Julien said. "It was a hit that was a bit from behind that deserved two minutes. It was probably just the way he went into the boards that gave us that result. But frankly, I don’t think it was anything more than that."
Pacioretty scored the second goal in the Canadiens' 2-1 win.