Williams missed Friday's workout and hasn't played or practiced since spraining his left ankle in the first quarter against Minnesota on Dec. 20.
Stewart returned to practice Friday after sitting out the past two days to rest his sore left Achilles tendon. Both have rushed for more than 1,000 yards this season.
Backup running back Tyrell Sutton missed Friday's shortened practice and also is doubtful with an ankle injury.
The Panthers signed linebacker Mortty Ivy from the practice squad to take injured receiver Steve Smith's spot on the 53-man roster.
Smith was placed on injured reserve Friday, five days after he broke his left forearm while catching a touchdown pass in a win over the New York Giants. Smith is the 12th player to go on IR for the injury-plagued Panthers.
Ivy is an undrafted rookie from West Virginia. He's been on the Panthers' practice squad since being in their final wave of cuts before the start of the season.