Southern California quarterback Mark Sanchez suffered an left knee injury early in the Trojans' practice seesion Friday afternoon.
According to The Press-Enterprise of Riverside, Calif., Sanchez was injured during a calisthenics session prior to practice.
The newspaper reported on its Web site Sanchez was injured during a carioca-shuffle drill - an exercise in which players move laterally, alternating their steps forward and backward.
Acording to Rivals.com Network site USCFootball.com , Sanchez left the field with the assistance of two trainers. He sat on a sideline table with ice on his left knee until he was carted into the athletics facility.
Backup quarterback Mitch Mustain – a transfer from Arkansas who began his college career in a 50-14 loss to the Trojans – assumed the starter's role in Sanchez' absence.
The No. 3 quarterback is redshirt freshman Aaron Corp.
Sanchez, a 6-foot-3, 225-pound junior, played in seven games last season - starting three of them in place of John David Booty. Sanchez threw for 695 yards and seven touchdowns in 2007.
Southern Cal has practiced this week in just shoulder pads and helmets - the Trojans won't don full pads until Sunday.