USC running back Tre Madden fell untouched while carrying the ball toward the end of Thursday's practice and appeared to seriously injure his left knee.
Madden, a sophomore who has been the talk of spring practice since switching over from linebacker last month, tried to make a cut 10 or so yards down from the line of scrimmage and immediately hit the grass. After spending roughly two minutes on the ground while being examined, he walked off the field with the help of the training staff and was then carted off the practice field.
Speaking after practice, Trojans coach Lane Kiffin said he did not know the severity of the injury but admitted he was worried it could be serious.
"There's that concern any time with knees when guys can't really walk afterward," Kiffin said.
Madden was not made available to speak with the media.