Walker will have an operation Friday in Houston and will be out "for a few weeks," coach Mike Shanahan said today.

"Javon's not feeling very good," Shanahan said.

Shanahan didn't have a precise timetable for Walker's return and said he wasn't sure what the procedure entailed.

"Doctors don't think it's very serious, but serious enough to go in there and clean it out because he's been off of it for a few weeks and it hasn't gotten much better," Shanahan said.

Walker underwent surgery two years ago to repair a torn ACL and then needed a follow-up operation around the time he joined the Broncos in a draft day trade in 2006.

Walker missed the Broncos' last two games because of swelling in his surgically repaired right knee but returned to practice this week following the team's bye.

Yesterday, Walker said he was feeling better and hoped to play Sunday night against Pittsburgh.

"I really don't notice it until everything is done and it's time to calm down," Walker said of the pain. "When I'm out there in the midst of playing, I don't even notice it."

Walker, however, wasn't at the team's complex today. After the day's workouts, Shanahan said Walker needed another operation.

Walker had a great training camp and said he was so confident in his recovery that he predicted he could top 1,600 yards receiving this season.

He got off to a great start, catching 17 passes for 220 yards in the first two games before making just two receptions for 10 yards against Jacksonville in Week 3. He sat out the next two games.

"He gave it a go, but you could tell something wasn't right," wide receiver Brandon Stokley said. "We're going to miss him. So hopefully he'll get back and be ready for the stretch run."

Walker's loss puts even more pressure on a Broncos offense that sputtered in a 41-3 loss to San Diego two weeks ago and lost center Tom Nalen for the season with a torn biceps.

Running back Travis Henry could be facing a year's suspension. He is fighting the league over a drug test result, and the Broncos could find themselves turning to undrafted rookie Selvin Young any day.