The Trojans are set to take on Louisville in the game on Dec. 27.
Williams, who threw for 3,633 yards and 30 touchdowns during USC's disappointing 7-5 season, is widely expected to forgo his senior season and declare for the NFL draft, where he is the projected No. 1 pick.
Last year, Williams broke out for 4,537 passing yards and 42 touchdowns, fueling USC's 11-1 regular season and securing USC's eighth Heisman Trophy.
With Williams' forthcoming departure to the NFL, sophomore backup Miller Moss is expected to handle starting quarterback duties during the Holiday Bowl. In three seasons at USC, Moss has thrown for 542 yards and three touchdowns in limited appearances. On Monday, Riley expressed that he wished he could have gotten Moss more snaps this season.
Riley added that he expects Moss to "fully be in the mix" for USC's starting job next season and that the program will also look to the transfer portal to potentially add "some quarterbacks."
"Not ready to say that we will for sure do anything," Riley said. "But we'll take a look and see how it all unfolds."
USC also has freshman and former five-star recruit Malachi Nelson on its roster, but Riley said Nelson had some "physical hurdles" this past season.