Chris Samuels has suffered a torn right triceps, an injury that ends his 2008 season.
Samuels will have surgery on the triceps, head coach Jim Zorn said. He will be placed on injured reserve.
"That's going to be it for Chris," Zorn said.
Samuels suffered the injury on the first play of the fourth quarter in Sunday night's 24-10 loss to the Baltimore Ravens. He was not able to return to the game.
Typically, a torn triceps requires a 10-12 week rehab. Randy Thomas suffered a torn triceps in 2007 and missed most of the season.
Samuels, a five-time Pro Bowl left tackle, should be fully healthy for the start of off-season workouts in the spring.
For now, the Redskins have three games left in the regular season and they are still in the mix for a Wild Card playoff spot in the NFC.
Who replaces Samuels for the stretch run?
The first choice is Stephon Heyer, who has backed up at both right and left tackle.
"I will help out any way I can," Heyer said. "I feel like I can give us an opportunity [to win and make the playoffs]."
Complicating matters is that Jon Jansen, the starter at right tackle, suffered a knee ligament sprain during the Ravens game. Jansen has not been ruled out for this Sunday's game against the Cincinnati Bengals, team officials said.
If Jansen is unable to play, then the Redskins are expected to turn to 11-year veteran Jason Fabini at right tackle.
Fabini played both right and left tackle the first nine years of his career, all with the New York Jets. He started 13 games at right guard for the Redskins in 2007.
Meantime, rookies Chad Rinehart and Devin Clark, expected to be signed by the club from the practice squad, will serve as backups at tackle.
Rinehart was the Redskins' third-round draft pick out of Northern Iowa last April. Clark was an undrafted rookie out of New Mexico.
Fabini and Rinehart were deactivated for Sunday's game against the Ravens, so the Redskins were forced to line up third-year lineman Justin Geisinger at left and right tackle.
Geisinger has played interior offensive line with the Redskins, but the team was in a bind with Samuels and Jansen both sidelined in the second half of the game.
It was the first time that Geisinger played offensive tackle since he was a senior at Vanderbilt in 2004.
"It has been a long time and I haven't taken snaps [at tackle] in quite a while," Geisinger said. "Circumstances happen when you're only dressing seven linemen and unfortunately both our tackles went down.
"I haven't played in a while, but that's part of being a backup in the NFL. Coaches expect you to fill a spot. You have to be ready for those situations. All I could do was go in there and do the best I could."
Geisinger was also diagnosed with a knee injury coming out of the Ravens game. He was at Redskins Park on Monday to receive treatment.
"[The injury] happened in the game," Zorn said. "His knee was sore from an earlier injury on a special teams play and this was just a reoccurring thing that happened when he had to go in and play."