Coach Kevin McHale said earlier Wednesday he was sure Harden would be "ready to go."
"We're confident coming in here. Obviously, we need James and we want James to play for us to win at a high level," teammate Chandler Parsons said.
"No doubt in my mind, he's going to play tonight. There's no way some little flu bug is going to keep him from playing in an elimination game."
The Rockets face a 3-1 deficit in their first-round series against Oklahoma City, after winning Game 4 to avoid elimination.
Harden was the NBA's fifth-leading scorer during the regular season but has struggled against his former team, committing 10 turnovers while scoring 15 points in Game 4.
"James, he's a tough player, a very good friend of mine. He's going to be fine," Patrick Beverley said. "He's going to push through it like he's supposed to, like a leader does."
McHale said point guard Jeremy Lin will be available only for spot duty off the bench if he plays at all in Game 5 because of a bruised chest. Lin was hurt in Game 2 before sitting out the second half. He was ineffective in Game 3 and then was held out of Game 4.
"It seems to be just kind of lingering in there, and he hasn't been able to do anything for three or four days," McHale said.
Lin said it will be up to the medical staff whether he's allowed to play.
"They've kind of taken away my freedom to choose whether I play or not, so I don't really have a choice anymore," Lin said.
Information from The Associated Press was used in this report.