"We kept Clinton at home just to get rest," Zorn said. "That was kind of doctor's orders."
Portis is having a disappointing season by his standards, with only 494 rushing yards. He last missed a game in 2006, when hand and shoulder injuries ended his season early.
Portis' injury means Ladell Betts will make his first start since 2006, assuming he is able to play. The backup is nursing a sprained ankle and didn't practice Thursday, although Zorn said he expects Betts to be available.
There also is considerable doubt as to the makeup of Washington's starting offensive line. Mike Williams is out with an ankle injury, so midseason signee Levi Jones will start at left tackle and Stephon Heyer has been moved from left tackle to right tackle. Heyer was limited in practice Thursday with a knee injury, and D'Anthony Batiste replaced him.
"That's kind of an ongoing battle," Zorn said. "Our ranks are running thin."
The Redskins' original starting left tackle (Chris Samuels) and right guard (Randy Thomas) are out with season-ending injuries.
Also, the Redskins are dealing with a now-weekly quandary at punter. Hunter Smith, who has a recurring groin injury in his kicking leg, will try to punt during Friday's practice. If Smith can't play, Sam Paulescu will punt for the second consecutive game and Smith will be a candidate for season-ending injured reserve.
"It's a possibility if he continues to reaggravate this thing," Zorn said. "Tomorrow is kind of an important day."
Copyright 2009 by The Associated Press