LOS ANGELES -- USC quarterback Cody Kessler said it was "terrifying" watching his teammate Marqise Lee needing to be escorted away from the practice field on Friday.
Lee missed the Trojans' first practice on Saturday, a day after he left practice early with an apparent right shoulder injury.
USC would not disclose the nature of Lee's injury, and the Trojans were scheduled to practice again later Saturday.
Lee went up for a deep pass over the middle, and with a defender draped over him landed awkwardly, suffering an apparent shoulder injury.
Lee was taken off of the field where soon after a school official carried his helmet and shoulder pads away from Howard Jones Field.
The reigning Biletnikoff Award winner was then escorted away through an alternate exit.
"He fell over there," said USC head coach Lane Kiffin. "They took him in to check him out and that's about all I know.
"(I know) nothing that would be completely factual that I would be able to report."
The injury occurred during the first 11-on-11 team drills of the day as the Trojans returned to practice in shorts and shoulder pads following their first scrimmage of the fall on Thursday.
Kessler threw the pass and saw Lee go down but, like Kiffin, wasn't sure the extent of the injury either.
"It's very, very terrifying but at the same time Marqise is a tough guy," Kessler said. "I actually talked to him when he was walking off (of the field) and he said 'Yeah, I'm fine.'
"When he was walking off (of the field) he said he was fine and everything felt good."
Once Lee left the practice field, the Trojans were left with three scholarship wide receivers. They entered camp with just five.
De'Von Flournoy missed his third straight practice on Friday with an undisclosed injury. 6-foot-6 walk-on quarterback Chris Willson lined up at wide receiver later during practice.
Lee caught 118 passes for 1,721 yards and 14 touchdowns in 2012 becoming the first Trojan ever to win the Biletnikoff Award. He owns or shares 22 school records. He is currently sixth on the USC career pass catching list with 191 career receptions. Former teammate Robert Woods owns the mark with 252.
-- The Associated Press contributed to this report
