Safety Kareem Moore is sidelined 4-6 weeks after undergoing arthroscopic knee surgery on Monday.
"Hopefully he'll be back sooner rather than later," head coach Mike Shanahan said.
Moore suffered the knee injury trying to make a tackle on a fake punt during last Saturday's preseason game against the Baltimore Ravens.
Prior to Monday's surgery, Moore was at Redskins Park and participated in a positional meeting, defensive coordinator Jim Haslett said.
"Oh, I feel sorry for him," Haslett said. "He's a great kid. He worked hard to get to that spot in his life and an opportunity to start, and this is a setback for him. But knowing him, he'll come back. It's going to take a little bit of time."
With Moore out, Reed Doughty is expected to get the first opportunity to start at free safety.
Chris Horton also could get reps at safety. Also, the team signed 10-year veteran Tyrone Carter, formerly of the Pittsburgh Steelers, to fortify the depth in the secondary.
Doughty has played in 45 games, with 16 starts, in his four-year NFL career. He has lined up mostly at strong safety, though.
He had his best season last year when he posted a career-high 93 tackles, two sacks and one interception.
Doughty said he always prepares as if he were a starter.
"There's no difference, if I prepared differently that means I wasn't preparing correctly before," he said. "I prepare like I'm the starter every day and when I'm a backup I'm always on the sidelines waiting."
Chris Horton, who also has played mostly strong safety in his career, said he would also take mental reps at free safety in case he is needed to play that position.
Horton played in the Ravens preseason game last Saturday, returning to action after being sidelined with a groin injury.
"I thought I did a good job running to the ball," Horton said. "I wanted to see where I was coming off that injury and I think I did pretty well."
-- KELLY STILL SIDELINED
Malcolm Kelly continues to be sidelined with a sore hamstring but he says he feels the injury is improving.
Kelly worked on the side with athletic trainers during Monday's practice.
"I did a whole lot better, it was night and day from last week," Kelly said. "We'll just have to see how it progresses."
Kelly's status for this Friday's preseason game against the New York Jets is uncertain.
Mike Shanahan said he was "keeping his fingers crossed" that the injury was not anything worse than a hamstring pull.
-- SELLERS UNCERTAIN FOR JETS GAME
Fullback Mike Sellers watched Monday's practice from the sidelines as he rehabs a knee injury suffered against the Ravens.
"It's bearable," Sellers told reporters. "I don't need to take any pain medication."
Sellers indicated he could play this Friday in the Jets preseason game, but he may be held out by athletic trainers.
The Redskins signed three-year veteran fullback Carey Davis, formerly with the Pittsburgh Steelers, to add depth at Sellers' position.
-- WILLIAMS BACK
Left tackle Trent Williams returned to practice on Monday despite suffering an elbow injury in the Ravens preseason game.
Williams, the Redskins' first-round draft pick last April, said he was looking forward to the Redskins' next preseason game to redeem himself.
He struggled in pass protection against linebacker Terrell Suggs, one of the game's best pass rushers.
"It was a huge learning experience," Williams said.