Eight weeks after he had surgery on his knee, Quinton Carter is still bothered by it. As a result, he won't play Sunday. (Andrew Mason)

Quinton Carter is still having knee trouble.

The second-year Broncos backup safety was ruled out for Sunday's game with the Raiders after not practicing all week because of a injury to the same knee on which he had arthroscopic surgery early in training camp.

"His knee's hurting," Broncos coach John Fox said.

Carter was limited in practice last week because of the knee injury, but still managed to play 29 snaps -- seven on defense and 22 on special teams.

Kuper unlikely to play: RG Chris Kuper practiced all week, but it appears as though the Broncos will carry out their plan to ease him back into game-time work, listing him as doubtful for Sunday.

Not only is Kuper recovering from a fractured left forearm he suffered Aug. 14, but he has not played in a game since fracturing his fibula and tearing ankle ligaments in the 2011 regular-season finale on Jan. 1. Those injuries had kept him from full-scale work during training camp; he was still easing back into a full workload when he suffered the arm injury.

With Kuper likely sitting out, Manny Ramirez is expected to make his fourth start of the season. It could scarcely go worse than last week's assignment, in which he allowed 2.5 sacks to Houston DE J.J. Watt.

Other injuries: LB Nate Irving (concussion) and WR Matt Willis (left hamstring strain) returned to practice on a limited basis; both are questionable. If Irving can't play, the Broncos might promote recently signed practice-squad LB Mike Mohamed to the active roster in order to have six healthy, available linebackers; they're already without suspended Joe Mays and D.J. Williams at the position. RB Willis McGahee (rib cartilage), TE Jacob Tamme (groin strain), CB Tracy Porter (knee bruise) and CB Chris Harris (ankle) all practiced fully and are listed as probable for Sunday.

For up-to-the-minute Broncos updates, follow Andrew Mason on Twitter at @CBSBroncos and @MaseDenver.