ASHBURN, Va. — Here is all you need to know to understand the tenuous nature of Washington's backfield.

Optimism was generated Wednesday because Clinton Portis was able to straighten his leg.

"It was hip, hip hooray," Redskins coach Jim Zorn said. "He's working hard to get back.

"It just remains to be seen if he can or not."

Portis suffered a second-degree sprain of the medial collateral ligament in his left knee in the loss to Pittsburgh 10 days ago. Zorn said it was, "pretty much a lock" that he wouldn't determine if the league's second-leading rusher (995 yards) can play until a few hours before his team faces the Cowboys on Sunday night.

Portis did not take the practice field Wednesday. Backup Ladell Betts did.

"It felt decent," said Betts, who hasn't played since spraining his left knee a month ago. "It's just a matter if I can go full speed on game day, whether I can take the hits and take the pounding."

If Portis and Betts are unable to play, the starting job will fall to Shaun Alexander. Rock Cartwright, who has only four carries this season, would also get some action.