Wide receiver Malcolm Kelly re-aggravated his hamstring injury during Monday's practice.
Head coach Mike Shanahan called the injury a "setback" but did not go into specifics.
"I'll let you know [on Tuesday] how serious it is but it looked like a pretty good pull," Shanahan said. "We'll wait and see."
Kelly did not talk to reporters after practice.
Kelly's status for the Arizona preseason game this Thursday night -- and the regular season, for that matter -- is uncertain.
It was thought by some that Kelly needed a good performance in the Arizona game to solidify a roster spot.
Kelly returned to practice on Monday for the first time in a month. He last practiced with the team on the second day of training camp before being sidelined with the hamstring injury.
He had an MRI on the hamstring as the injury lingered through training camp.
After weeks of rehab, Kelly and the team's athletic trainers believed the hamstring was healthy enough for him to go full speed at practice.
"Obviously it must be a lot worse than we first thought," Shanahan said.
Kelly was one of three 2008 second-round draft picks by the Redskins.
In his two-year NFL career, Kelly has started 9-of-21 games and caught 28 passes for 365 yards.