Hill attempted to make a catch across the middle during an afternoon practice in intense heat, but came down wrong on his leg and fell to the ground screaming in pain.
Trainers taped Hill's knee before helping him on to a cart. Hill left the field with his head in his hands and a towel draped over his head.
Panthers assistant coach Steve Wilks said after practice he had no immediate word on the extent of the injury and didn't want to speculate.
''I saw Hill go up in the air and come down on his knee and possibly twist it a little bit, but I don't want to say anything until I have a chance to meet with (the trainers),'' said Wilks, who was filling for coach Ron Rivera.