"He'll stay overnight (at Montreal General Hospital) and we'll see how he feels," said head coach Guy Carbonneau. "It's a pretty bad concussion."
Downey crossed the red line with his head down and was dropped by a thundering shoulder check from Regehr 3:38 into the Canadiens 5-4 victory at the Bell Centre.
He lay motionless on the ice for a minute, but later was able to get up and looked wobbly as he was helped off the ice.
"I wanted to try to make a good clean hit on him," said Regehr, who was not penalized. "He was trying to get the red line to make a dump-in and I saw my opportunity to close in on him.
"I just put the shoulder to him. I hit him right on the button. I hope he's not hurt seriously. For me, in my position, I'm supposed to be physical. I'm pretty sure it was clean."