Allen is the third starting defensive back to suffer a season-ending injury. Cornerback Wil Peterson (stress fracture in lower back) and safety Shaun Williams (knee) were previously placed on IR.
Fassel is hopeful that cornerback Ralph Brown, who has missed three games with a dislocated shoulder, will be able to return for the Redskins game.
"The prognosis is he will play Sunday," Fassel said. "But I also know that when a guy is coming back from a shoulder (injury), he is not coming back 100 percent. Ralph is a tough guy and I think he will be ready to go."
Guard Wayne Lucier strained a knee ligament and will be sidelined for three to four weeks. Tight end Marcellus Rivers suffered the same injury and is expected to miss two or three weeks.
Fassel did not know if left tackle Luke Petitgout, who missed Sunday's game with a back injury, will be well enough to face Washington. That leaves the offensive line, which had a rough day against Buffalo, in a state of flux.
Jeff Roehl, who played for Petitgout against the Bills, might stay at left tackle. Or Fassel might decide to give Jeff Hatch, last year's third-round draft choice, a shot. Hatch missed all of last year with a back injury and has not played a down this season.
"One of the options is Roehl, one of the options is Hatch, one of the options is to move some people," Fassel said.
Scott Peters, who joined the team as a practice squad center and found himself playing left guard when Lucier went down, will presumably be in the mix up front.
Jeremy Shockey had an MRI today on his injured knee. Fassel was noncommittal about Shockey's availability for the Redskins game.
"We'll probably go through the week and see how he feels," Fassel said. "The key thing is to make sure he doesn't have any swelling in that knee. That is the starting point."
Will Allen placed on injured reserve
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