Their status for this Saturday's game against Stanford is unknown. Horton came out at halftime -- or, rather, after the hour-long halftime weather delay -- because of an unspecified ailment and Holmes was carted off the field in the fourth quarter because of an apparent ankle injury.
USC does not discuss injuries per a new policy instituted last month, and whether players practice on a given day cannot be disclosed by the media, per the policy.
But Trojans coach Lane Kiffin said Thursday that Cyrus Hobbi, the Trojans' backup center, has taken "some reps at guard and center" this week, as has senior Abe Markowitz, who was Holmes' primary backup last season.
"Cyrus has done some good things," Kiffin said. "So we have great confidence no matter who goes in."
Holmes started his USC career as a guard before moving to center in the spring of 2011, when Kristofer O'Dowd graduated. Kiffin said that has helped him be an asset to the team on the offensive line.
"Great leadership," Kiffin said when asked Holmes brings to the team. "Extremely intelligent player. I think playing guard early has helped him so he basically knows what everybody does."
Markowitz exited MetLife Stadium on Saturday with a sling on his left shoulder, but Kiffin said he has practiced this week.