PITTSBURGH — Steelers running back Willie Parker was unexpectedly held out of practice Wednesday after aggravating the sprained medial collateral ligament in his left knee.

Parker, who hasn't played since Sept. 21, practiced Monday and was expected to play Sunday in Cincinnati, but his return is now in doubt.

Parker deferred questions about his injury to coach Mike Tomlin, who did not talk to reporters Wednesday. However, Parker said he is frustrated by the injury flare-up.

"We're going to take it day by day," Parker said.

Parker aggravated the injury during Monday's workout. The Steelers (4-1) normally don't practice that day, but did so this week because they had a bye last weekend.

Also missing practice Wednesday were left tackle Marvel Smith (back spasms) and four-time Pro Bowl nose tackle Casey Hampton (groin). Hampton, who has missed two games and most of a third, may return to practice Thursday.

Parker, a Pro Bowl running back the last two seasons, ran for 138 yards and three touchdowns against Houston and 105 yards against Cleveland in Pittsburgh's first two games. He was held to 20 yards on 13 carries during the 15-6 loss to the Eagles in which he was hurt.

First-round draft pick Rashard Mendenhall replaced Parker and made his first NFL start Sept. 29 against Baltimore, but was lost for the season with a shoulder injury after gaining 30 yards on nine carries.

With none of the Steelers' top three running backs healthy — fullback Carey Davis has a sprained ankle — third-down back Mewelde Moore made his first start since 2005 and gained 99 yards against the Jaguars on Oct. 5.

If Parker can't go, Moore probably would start against Cincinnati, with Najeh Davenport and Gary Russell backing him up. Davenport and Russell, who spent the 2007 season with Pittsburgh, returned after Mendenhall was hurt.

Parker was leading the NFL with 1,316 yards rushing last season when he broke his right leg Dec. 20 against St. Louis. With Parker out, the Steelers lost their final regular-season game in Baltimore and their wild-card playoff game against Jacksonville.