Helton did not provide a specific reason, only that it was "a violation of team rules," while appearing on his radio show on ESPN LA 710.
Masina appeared in 12 games as a true freshman last season and finished with 25 tackles. He returned a fumble 46 yards for a touchdown in the first game of the season.
USC's depth chart lists Masina as a co-backup to Michael Hutchings at inside linebacker with senior Quinton Powell. The Trojans' other starting linebacker is sophomore Cam Smith, who is backed up by junior Olajuwon Tucker.
Helton also announced sophomore Chuma Edoga will start at left tackle in place of second-team All-Pac-12 selection Chad Wheeler, who has been limited in practice due to plantar fasciitis. USC will monitor Wheeler's progress through the week and Helton allowed for the possibility he could see some playing time. Edoga started two games at right tackle last season.