USC would not disclose the nature of Lee's injury, and the Trojans were scheduled to practice again later Saturday.
The All-American appeared to get hurt while attempting to catch a long pass from Cody Kessler in coverage during Friday's work outs. He stayed down on the field before USC's medical staff escorted him away.
Kessler said Lee told his teammates he would be "fine" before he left the field.
Lee had 118 receptions for 1,721 yards and 14 touchdowns last year as a sophomore, winning the Biletnikoff Award as the nation's top receiver. He also was the Pac-12's offensive player of the year.
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