Pacioretty left at 4:04 of the third after he fell headfirst into the boards on a check by Anaheim defenseman Clayton Stoner. No penalty was called.
"It's an upper-body injury and we sent him to the hospital for precaution," Canadiens coach Michel Therrien said after the 2-1 loss.
Pacioretty leads Montreal with 13 goals and 25 points.
"I think their team wasn't happy about it," Stoner said. "I didn't mean to hurt him but the game's fast and sometimes guys go into the boards wrong, so I hope he's all right and I didn't mean any intentions to hurt him."
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