LANDOVER, Md. -- Washington Redskins running back Clinton Portis was unable to practice Monday because of a sprained knee, and coach Jim Zorn said Portis is "50-50" for Sunday night's game against the Dallas Cowboys.

Portis bruised his knee on the final play of the first quarter of last week's 23-6 loss to the Pittsburgh Steelers but returned for the next series and finished the game. The pain got progressively worse after treatment following the game, however, and Zorn said Monday that Portis has a bone bruise on the femur as well as a sprain.

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