Running back Clinton Portis is unlikely to play in next Sunday's game against the Denver Broncos, while right tackle Mike Williams is definitely out.

Portis suffered a concussion in the Redskins' 31-17 loss to the Atlanta Falcons and Williams had a ligament tear in his ankle.

Portis suffered the concussion as he was tackled early in the first quarter of the Falcons game.

Jim Zorn said on Monday that Portis was still experiencing headaches and that it was "doubtful" he would play in next Sunday's game against the Denver Broncos.

"That's just my opinion, but I know we have to be very cautious with a concussion," Zorn said.

Ladell Betts is expected to start in place of Portis. Betts sprained his ankle in the Falcons game, but the injury is not considered serious.

The Redskins could explore signing former Kansas City Chiefs running back Larry Johnson, who was waived on Monday. If he clears waivers, then he would be an unrestricted free agent.

Zorn said the team has had internal discussions about adding Johnson to the roster.

If not Johnson, then the team may re-sign running back Quinton Ganther.

Ganther was on the Redskins' roster last week, but he was released when the team needed to sign punter Sam Paulescu.

Williams is expected to be sidelined 2-4 weeks.

"It's not going to require surgery," Zorn said.

With Williams out, the Redskins are expected to shift Stephon Heyer back to right tackle and insert Levi Jones at left tackle. Heyer started the first four games at right tackle.

Jason Campbell had a low ankle sprain--not a high ankle sprain as was originally reported by Zorn--and should be available to play against the Broncos.

"I suspect he should be fine and ready to play," Zorn said.