Trojans coach Clay Helton said Monday that Stepp will need surgery to repair a torn ligament and is out at least three weeks.
Stepp, a redshirt freshman from Indianapolis, has rushed for 307 yards and three touchdowns in six games this season.
The 6-foot, 235-pound Stepp provided a physical presence to complement the spread passing component of the Air Raid offense in the previous three games before he was hurt Saturday in the third quarter of USC's 41-14 win over the Wildcats.
Stepp's injury is the latest blow to USC's depth at running back. Redshirt junior Vavae Malepeai, who leads the team with 406 yards rushing and four touchdowns, remains out after undergoing knee surgery last week. Junior Stephen Carr did not practice Monday after sustaining a hamstring strain against the Wildcats.
Helton did not know if Carr, who has 266 yards rushing and two touchdowns, would be available to play Friday at Colorado.
"I've never lost three backs in one week in 25 years before, but, you know what, there's a first time for everything," Helton said.
Those injuries leave freshman Kenan Christon in line to start against the Buffaloes. Christon, the 2019 California high school 100-meter champion, had touchdowns runs of 55 and 30 yards against Arizona and finished with 103 yards rushing on eight carries.
Quincy Jountti, a former walk-on who earned a scholarship in August, and Dominic Davis, a redshirt senior who moved back to offense after playing cornerback, were the two other scholarship running backs who practiced Monday.
USC is tied with No. 12 Utah for the lead in the Pac-12 South with a 3-1 record, but the Trojans hold the potential tiebreaker because of their 30-23 win over the Utes on Sept. 20. USC is 4-3 overall.