Portis, who has rushed for a team-high 195 yards on 49 carries this season with two touchdowns, was injured Sunday during his team's 17-12 win over the Philadelphia Eagles.
An MRI scan revealed Portis, who missed eight games last season after a concussion, has a third-degree separation of the groin, according to the team's website (http://www.redskins.com/).
"Hopefully he's back in four weeks instead of six weeks, but a third degree separation is off the bone. It's quite sore right now," said Redskins coach Mike Shanahan.
"When you lose some players during the season, other guys have to step up."
The Redskins are expected to start Ryan Torain at running back when the team plays the Green Bay Packers on Sunday.
(Reporting by Frank Pingue in Toronto; Editing by Julian Linden)