USC appears on track to play its opener against Fresno State with a depleted running backs corps.
Tre Madden was absent from practice again Friday because of a toe injury.
"When you start messing with a running back and his feet and his toes and the ability to push off, they don't just overnight all of a sudden get better," Sarkisian said.
Madden has not practiced since Monday night when the Trojans scrimmaged at the Coliseum.
He saw a foot specialist Thursday and was scheduled to see another Friday, Sarkisian said.
Sophomore Justin Davis is working as the No. 2 tailback behind Javorius Allen. Fullbacks Soma Vainuku and Jahleel Pinner also have played tailback.
Madden started the first six games last season before suffering a hamstring injury. He finished with a team-high 138 carries and rushed for 722 yards and three touchdowns.