NFL.com's Ian Rapoport reported Monday that the second-year back had an MRI on his right knee and "he's likely to miss some time." The team is hoping that he won't need surgery but will give him a full evaluation after he rushed for only 18 yards on 10 carries in Sunday's loss to the Philadelphia Eagles.
Guice missed his entire rookie season after tearing the ACL in his left knee in a preseason game last August.
Guice's likely absence would give Adrian Peterson the opportunity to carry the load after he was a surprise inactive against Philadelphia.
Peterson led the Redskins with 1,042 yards last season, when he scored seven times. Chris Thompson and Wendall Smallwood also should see carries against the Cowboys.