Tennyson fell backward and hit his head on the ice in the first period. He dropped flat on his back at 8:37, before he was able to get to his feet and was helped off the ice by teammates and Sharks trainer Ray Tufts. He appeared to have a cut on his head.
Reaves was given a five-minute boarding penalty and a game misconduct. San Jose turned the sequence into two power-play goals.
"I think that's the ultimate retribution for a hit like that, is to score two or three power-play goals, and that decision basically cost them the game,'' San Jose coach Peter DeBoer said. "I think Matt's OK. He's not great, obviously. He's probably concussed and we'll see what else. Those are the kind of hits we're trying to get rid of, so I'm sure the league will deal with it.''
St. Louis defenseman Robert Bortuzzo said the hit by Reaves was not intentional.
"The game is moving real quick out there," Bortuzzo said. "Reaves plays the game hard and on the edge, but he's never been known to be malicious or anything. It's a tough situation for a guy to go down.''
Information from The Associated Press was used in this report.