CHARLOTTE, N.C. -- For much of the past two years -- even much of training camp this year -- Jonathan Stewart was the Carolina Panthers' running back that spent most of practice riding the stationary bicycle and working with the trainer.

Talk about role reversals.

On Wednesday, Stewart and practice squad player Darrin Reaves were the only healthy backs with DeAngelo Williams (thigh), Mike Tolbert (chest) and Fozzy Whittaker (thigh) unable to participate.

On Thursday, Tolbert returned on a limited basis, but Williams and Whittaker still were held out.

"Yesterday seemed like the roles switched," said Stewart, plagued by ankle injuries the past two seasons and a hamstring injury at the start of training camp. "Usually, I'm that guy on the bike."

While coach Ron Rivera was optimistic Williams would be available for Sunday night's prime time game against Pittsburgh, Tolbert remains a question mark.

Rivera said Tolbert still isn't in position to take a hit after taking a blow to the rib area during Sunday's 24-7 victory against Detroit. Tolbert called the hit from defensive tackle Nick Fairely the hardest he's taken in seven NFL seasons.

Rivera hasn't ruled out the possibility that Reaves could be brought up from the practice squad to fill defensive end Greg Hardy's roster spot.

The team's 2013 sack leader on Wednesday was placed on the commissioner's exempt list until his domestic violence case is resolved.

Rivera also didn't rule out calling up a wide receiver, Stephen Hill or Marcus Lucas, if a lingering thigh injury sidelines Jason Avant. Hill played the past two seasons with the New York Jets before being released.

Avant went from limited in Wednesday's practice to not participating.