WASHINGTON — Redskins running back Clinton Portis is “very, very doubtful” for Sunday’s National Football League game against Denver as he continues to feel the effects of a concussion.

“I don’t want to rule him out, but it’s very doubtful that he’s going to play,” Redskins coach Jim Zorn said. “I would say ‘very, very doubtful.’”

Portis departed in the first quarter of the Redskins’ 31-17 loss to Atlanta on Sunday after a helmet-to-helmet hit.

“He would play if he could, but he really is sort of mellowed out with this concussion. Just a little quiet. He needs to get rest,” Zorn said. “And I think it’s a dangerous thing. I would never say, ‘Come on, Clinton! You can do it!’ That wouldn’t be the right thing to try to push, and we’re not going to do that.”

Portis ranks eighth in the NFC with 494 yards rushing this season for the Redskins, who have lost four straight games.

The Redskins also said Wednesday that strong safety Chris Horton had toe surgery and was put on injured reserve, ending his season.