"The time is right," McKnight said. "This was my first healthy season and I really don't want to come back and risk injury."
The Trojans held McKnight out of the Emerald Bowl against Boston College in December amid an ongoing investigation into whether he violated rules by using an SUV that does not belong to him, as well as his relationship with the vehicle's owner, Santa Monica businessman Scott Schenter.
McKnight was not cleared in time by USC's compliance department to participate in the Emerald Bowl, a 24-13 victory for USC. He said Friday that the ongoing investigation did not factor into his decision to leave school early.
"If I would have come back it would have been to pursue the Heisman," said McKnight, who also spoke of the difficulty of being compared with former USC running back Reggie Bush.
"My career was OK," McKnight said. "It was good, but not like Reggie. There was a constant pressure to be like Reggie."
McKnight was USC's first 1,000-yard rusher since 2005. He gained 1,014 yards, scored eight touchdowns and averaged 6.2 yards per carry. The Trojans finished 9-4, their worst season since 2001. The Trojans failed to make a BCS bowl and win the Pac-10 title for the first time in eight years.
Joe Schad covers college football for ESPN.