McGloin was injured on a late hit by Broncos defensive end Jared Crick with less than two minutes left in the first half. Cook, the Raiders' fourth-round draft pick out of Michigan State, made his NFL debut after not being active all season and he completed two of his first three passes for eight yards.
McGloin, meanwhile, was just six of 11 for 21 yards as the Raiders trailed, 17-0, at halftime. He was originally ruled "questionable" to return as he went to the locker room after being worked on by trainers on the sidelines.
With the loss, Oakland now falls to the No. 5 seed in the AFC playoffs and will play at the Houston Texans (9-7) as a wild card next weekend. The Kansas City Chiefs, after a 37-27 victory over the San Diego Chargers, secured the AFC West crown and the No. 2 seed in the conference with a first-round bye and play host to a playoff game.