Thornton appeared to hurt his shoulder when he was hit by Raffi Torres in the third period of the Sharks' Game 4 loss on Sunday.
He didn't practice Monday, but with the Sharks facing elimination with a 3-1 series deficit against the Canucks, Thornton said earlier Tuesday that he would play after taking part in the morning skatearound.
"One hundred percent," Thornton said. "Whatever it is, it's fine today. It's tough missing the last 10 minutes of any game, but I feel fine and ready to help out the guys as much as I can. All the guys realize if you don't win tonight you go home."
Thornton leads the Sharks with 17 playoff points. He also plays against the opposition's top forwards, and leads San Jose with 318 faceoffs, winning 59.4 percent, which may be difficult with a sore shoulder.
"He's played extremely well, he's our leader," Sharks coach Todd McLellan said. "I think we're going a little overboard as far as him being injured, so I'll leave it at that."
San Jose defenseman Jason Demers, out since Game 7 of the conference semifinals against Detroit, said h was ready to return but was a scratch with an upper-body injury.
Canucks defenseman Christian Ehrhoff, knocked out of Game 3 with a suspected shoulder injury, did not skate Tuesday morning after practicing Monday. He did not play and is still listed as day to day.
Information from The Associated Press contributed to this report.