Clinton Portis will be sidelined 4-6 weeks after an MRI revealed that he suffered a third-degree separation of the groin.
Head coach Mike Shanahan revealed the news following Wednesday's practice.
The injury will not require surgery, Shanahan said.
Shanahan characterized the injury as a "temporary setback" for Portis.
"Clinton is tough," Shanahan said. "He'll get a lot of rehab here. 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."
Portis suffered the injury in last Sunday's game against the Philadelphia Eagles.
Ryan Torain is expected to start in this Sunday's game against the Green Bay Packers. He said he took all of the first-team reps in Wednesday's practice.
Torain had a strong game against Philadelphia, rushing for 70 yards and a touchdown on 18 carries.
"When you lose some players during the season, other guys have to step up," Shanahan said. "Obviously, Ryan – he has had a little playing time and we'll see what he can do. And Chad [Simpson], we'll get a chance to evaluate him. Those two guys will start off as one and two."
Shanahan said the team would go into Sunday's game vs. Green Bay with three running backs.
That means a roster move is coming.
"We're always looking," Shanahan said. "We have a few directions we could go. We're still evaluating exactly what we will do."
Asked if re-signing Larry Johnson is a consideration, Shanahan replied: "We have a few options relative to people who have been here and who are out there [as free agents]."
Portis leads the Redskins' offense with 195 rushing yards and two touchdowns on 49 carries. He also has four catches for 41 yards.
It's the second year in a row that Portis will miss time with a significant injury.
Last year, Portis suffered a concussion midseason and sat out the final eight games.