The injury doesn't appear serious and Williams was joking with his teammates as he rode a stationary bike Wednesday. Coach John Fox wouldn't say which knee it was or when Williams started experiencing pain.
The Panthers were without their top two running backs. Jonathan Stewart missed his 14th straight practice with pain in his left Achilles tendon.
Williams and Stewart combined to rush for 2,351 yards last season, the highest total for teammates in the NFL in 24 years.
Linebacker Jon Beason (knee), safety Charles Godfrey (hand) and safety Nate Salley (knee) also sat out. Tackle Travelle Wharton was excused to deal with a personal matter.