Bills RB Karlos Williams (shoulder) in a sling Thursday

Published Dec. 3, 2015 2:32 p.m. ET

It's been an up-and-down season for Buffalo Bills rookie running back Karlos Williams. When healthy, he's been one of the best rookies in the league, even as a fifth-round pick and LeSean McCoy ahead of him on the depth chart.

Unfortunately, he's had trouble staying on the field as he's dealt with a concussion and now a shoulder injury. Williams left Sunday's game against the Kansas City Chiefs after injuring his shoulder and has yet to participate in practice this week.

On Thursday, Williams was spotted with a sling on his left arm. Rex Ryan said it doesn't look good for him being available against the Houston Texans, adding that the Bills would "make a move" to add to their running back depth if he's unable to go.

Williams was a touchdown-scoring machine earlier in the season, finding the end zone in each of his first five games. Injuries have derailed what was a promising rookie year, though there's plenty of reason to be excited about the young running back going forward.