Williams was limited all week because of a shoulder injury and is not dressed to play.
Emerging as one of the top defensive linemen in college football this season, the 6-foot-5, 290-pound sophomore from Daytona Beach, Fla. has a team-high 11 tackles for loss this season and is tied for the team lead with 56 tackles.
USC already ruled out outside linebacker Morgan Breslin for the rest of the regular season earlier this week because of a hip injury.
Redshirt junior J.R. Tavai, who had been playing well in Breslin's absence, is listed as the backup to Williams at defensive end in the Trojans' 5-2 defense. Redshirt freshman Jabari Ruffin would likely start at outside linebacker.