Pittsburgh Steelers wide receiver JuJu Smith-Schuster is not expected to play Sunday night against the Buffalo Bills after he aggravated his knee injury during practice on Thursday, a source told ESPN's Adam Schefter.

Smith-Schuster returned to practice Wednesday as a full participant, but he left early on Thursday and was listed as a limited participant in the team's official injury report.

On Thursday, offensive coordinator Randy Fichtner was still optimistic about Smith-Schuster's return, adding that the third-year receiver was eager to get back on the field.

"He was wanting to get back last week," Fichtner said. "You know, injuries. He's got all the enthusiasm and energy and effort and want to. You know, sometimes injuries take their own time. You've just got to keep working through it."

Smith-Schuster has missed the past three games with the knee injury and had been eyeing a return Sunday night. He only returned to running in a straight line on Dec. 4.

Smith-Schuster was injured Nov. 14 against the Cleveland Browns. He hurt his knee and suffered a concussion on the same play.

Running back James Conner was injured in the same game. He has an injury to the AC joint in his shoulder, but was a full participant in practice on Wednesday and Thursday.

In the absence of Smith-Schuster and Conner, the Steelers turned to a committee of offensive skill players to fill their roles. Wide receivers Diontae Johnson and James Washington stepped up in the past two weeks, and the Steelers deployed four different running backs last week against the Arizona Cardinals.

The Steelers (8-5) are on a three-game win streak without Conner and Smith-Schuster, but coach Mike Tomlin was blunt and succinct during his Tuesday news conference when asked how much the offense needed the pair of playmakers.

"A lot," he said.