Pacioretty was trying to get past the six-foot-nine defenceman when he was hit hard and went head-first into the glass partition between the benches with 15.8 seconds left in the period.
He lay motionless for several minutes and had his eyes closed as he was lifted onto a stretcher and taken off the ice.
A team spokesman said Pacioretty was conscious, had movement in all his extremities and was taken to hospital for observation.
The joyful mood in the arena evaporated instantly.
A capacity crowd, which moments earlier had been humming with easygoing exchanges between Canadiens fans and the Bruins fans in attendance, was gripped in an awkward silence.
Security guards in one section intervened to stop a developing altercation, when a livid Canadiens fan turned to a man in a Bruins jersey and screamed, "Are you proud to be wearing that?"
Chara was given a five-minute major for interference and a game misconduct.
Montreal led 4-0 at the time.