Pryor, who leads all NFL quarterbacks with 504 yards rushing, initially injured his knee in the second half of Oakland's 49-20 loss to the Philadelphia Eagles on Nov. 3.
He convinced the coaching staff to let him play the following week against New York and wore a thick brace over his knee, though his mobility was clearly not the same. The Giants held Pryor to a season-low 19 yards on the ground and registered four sacks while sending the Raiders to their third loss in four games.
Pryor was held out of practice Wednesday initially as a precaution. He returned Thursday, albeit in a limited fashion, then was downgraded 24 hours later.
If McGloin starts, it would cap a meteoric rise after the former Penn State quarterback began training camp fourth on the depth chart. He steadily climbed the ranks, however, and moved into the backup role after the Raiders released Matt Flynn on Oct. 7.
McGloin made his NFL debut midway through the fourth quarter against the Eagles and completed 7 of 15 pass attempts for 87 yards.
"Obviously, he's taken most of the reps to this point on the show team, but he's done a great job," Allen said. "He just has a little bit of that kind of 'it' factor about playing the quarterback position. He's got a lot of confidence in himself, and we've got a lot of confidence in him."
The 23-year-old McGloin was a walk-on at Penn State, the first to start at quarterback for the Nittany Lions in more than 60 years. He holds the school record with 45 career touchdown passes and had more 300-yard games than any other Penn State quarterback.
McGloin was bypassed in the draft, however, and the Raiders extended him an invitation to training camp only to keep the other quarterbacks on the roster from getting overworked.
Now he's on the cusp of making his first NFL start.
"I prepared all week to start, and I feel like I've prepared for the last eight or nine weeks to start," McGloin said. "Definitely felt great getting a lot of reps out there, being able to see some things that you normally wouldn't see. It's not my decision but if needed I'll be ready."
Allen said he doesn't expect the Raiders to have to alter their offense much if Pryor is held out.
"With Matt in there we're probably not running as many read option plays and that type of stuff," Allen said. "But the basis of the offense and what we do is not a whole lot different."
RB Darren McFadden will miss his second straight game and third this season because of a hamstring injury.
First-round draft pick CBD.J. Hayden (groin) has also been ruled out.
RT Tony Pashos did not practice because of an illness but is expected to play.