Allen was a limited participant in practice Friday, his first participation this week after suffering a right elbow injury Sunday against the New York Jets. He was seen at the end of the session wearing a practice jersey for the first time this week. Allen did not have on the sleeve he had been wearing on his right arm.
Sources told ESPN's Chris Mortensen on Monday that Allen was dealing with an injury to the ulnar collateral ligament and related nerves.
Bills coach Sean McDermott said Friday before practice that the team was taking a measured approach with its star quarterback.
"We are literally in an hour-to-hour situation here," McDermott said, "just looking at how he's going to progress through the day and as he goes through his different tests medically, making sure that he's able to check the boxes and the progression to where we can see if we can proceed through the course of the day here."
Allen was not present during the brief portion of practice open to the media Friday, and McDermott has declined to say what his work has been behind the scenes. Allen has been out at practice during the portion not open to the media, helping his teammates on the field.
"I mean, of course, Josh can be a little down," wide receiver Gabe Davis said. "I mean, he is a competitor, he wants to play, wants to be healthy. It sucks when injuries happen, but he's positive. He's out there coaching and helping us with things that he knows and helping Case [Keenum], and we're all just retaining all that information and getting ready to go."
Allen has not missed a game since his rookie year in 2018, when he sat out four also because of a UCL injury in his right elbow.
If Allen cannot play, Keenum would make his first start with the Bills after the team acquired him in the offseason from the Cleveland Browns. Keenum has played this year only with games out of hand, completing 2 of 7 passes. Keenum, who played for the Vikings in 2017, went 2-0 as a Browns starter last season.
"I think we planned for this potentially, right from the start," McDermott said. "And so we'll see where it goes. I think the other thing you have to layer in now is potential for weather come game day. And here we go with that in terms of that could be the case for the rest of the season. So we do the best we can.
"At the end of the day, Josh needs to be Josh, if he's able to play. And if Case plays, then Case needs to be Case and run the offense."
Allen was injured during the final drive against the Jets when defensive lineman Bryce Huff hit his throwing arm, resulting in a strip-sack fumble recovered by Buffalo. Two plays after the hit, Allen threw a 69.3-yard pass intended for Davis -- the longest pass attempt over the past six seasons, according to NFL Next Gen Stats -- but it fell incomplete and ended the game.
The quarterback has accounted for 82% of the Bills' total yardage and 92% of their offensive touchdowns, both the highest in the NFL.
The Bills' offense, which has not scored a passing touchdown in the past six quarters, might change a bit with Allen not in the lineup, but big overhauls are not expected with Keenum on the field.
"For the most part, [the offense] doesn't change," wide receiver Isaiah McKenzie said. "Case is going to go out there and throw the ball, throw the ball to his guys, and he's going to have fun. I mean, he probably can't run as good as Josh, but I'm pretty sure if he had to get into open field, he'll make a couple moves. ... I feel like I'm comfortable with Case. I'm pretty sure everybody else is comfortable with Case, and if Case is playing on Sunday, then that's what we're going to roll with."