LOS ANGELES (AP) Southern California running back Kenan Christon showed he can provide big plays, scoring on touchdowns runs of 55 and 30 yards in a memorable debut against Arizona.

Now the freshman with elite speed will have to show he can handle a bigger role, as Christon is expected to start for the Trojans at Colorado on Friday because of a spate of injuries at the position.

Redshirt freshman Markese Stepp is out up to five weeks because of an ankle injury he suffered on Saturday against the Wildcats that will require surgery, coach Clay Helton said. Junior Stephen Carr did not practice Monday after sustaining a hamstring strain in the same game. Redshirt junior Vavae Malepeai remains out after undergoing knee surgery last week.