The good news is he should be back in time for the playoffs.
Harden passed the NBA-mandated tests required for him to return from the concussion he suffered when the Lakers' Metta World Peace elbowed him in the back of the head, but the Thunder held him out of Wednesday night's regular-season finale against the Denver Nuggets.
"We decided as a group to hold him out one more game and give him 24 more hours," coach Scott Brooks said. "He should be fine in our recovery practice tomorrow as a team and our practice Friday."
The playoffs start this weekend, with Oklahoma City as the No. 2 seed in the Western Conference and hosting its postseason opener.
"I'm just worried about his health and want to see him back on the floor with us because he means so much to us," said Durant, the league's two-time scoring champion. "He means so much to the city.
"It's just tough not to see him right next to me in the locker room."
Durant and Harden have neighboring lockers in the dressing room and the two have grown into close friends, now even sharing an agent.
When World Peace, formerly known as Ron Artest, reared back and clocked Harden after a dunk in the first half Sunday, it hit Durant hard emotionally.
"Maybe if I would have stopped Ron Artest from dunking that ball, he wouldn't have did it. So I kind of felt a little bit of me when he did it," Durant said.
"I'm just looking forward to seeing him back in the locker room."
World Peace was suspended seven games for elbowing Harden. He'll miss the Lakers' regular-season finale Thursday night at Sacramento and then the start of the playoffs.
With Harden being cleared, it removes the concern that World Peace could beat Harden back to the court if his concussion had been severe.
"James is a strong person, he's a positive person and he doesn't let anything like that bring him down," Durant said. "And he didn't bring that into the locker room. ... From seeing him yesterday, you would think it never happened."
Harden averages 16.8 points, easily the most among NBA reserves, and was two games removed from his first career 40-point outing when he was clobbered by World Peace.
Oklahoma City beat Sacramento 118-110 on Tuesday without him and is 3-0 in games Harden has missed -- all against the Kings and New Orleans, the two teams at the bottom of the West standings.
"You don't win in this league without having your best players," Brooks said. "You can do it for a short period of time. You need your best players.
"James has been having a terrific year. He's been one of the leaders on our team. He's been consistent. He gives us a spark, whatever we need off the bench -- whether it's scoring, playmaking, defending. He's been terrific. He's a really good player."