Allen's start comes after Nathan Peterman started the Bills' first preseason game and AJ McCarron started the team's second preseason game. The team has yet to name a starter for its regular-season opener against the Baltimore Ravens.
McCarron suffered a right shoulder injury during his start Friday, but tests revealed he did not suffer a fractured collarbone, a source told ESPN's Adam Schefter on Monday evening. McCarron is expected back to practice soon, the source told Schefter.
Bills coach Sean McDermott said Sunday that the team would "pick up where [it] left off" with its three-way quarterback competition involving McCarron, Peterman and Allen if McCarron is healthy.
The Buffalo News reported after Friday's game that McCarron suffered a hairline fracture in his collarbone, but McDermott said Sunday that McCarron was seeking a second opinion. McDermott also said Monday that he and general manager Brandon Beane were discussing adding another quarterback to their roster, but it is unclear if that option is still being explored, given McCarron's expected return.
Allen took first-team reps in Monday's practice after Peterman led that group in Sunday's practice. The Bills have rotated their quarterbacks since practices in the spring, with all three seeing time with the top offensive unit.
"We're staying with the rotation," McDermott said Monday. "Nate had the ones [Sunday]. Josh will have them [Monday]. We're just with the rotation at this point."
Peterman started the preseason opener and played almost the entire second half of the Bills' second preseason game Friday. He has completed 17 of 20 passes this preseason for 231 yards, two touchdowns and one interception.
McCarron played in the second quarter of the Bills' first preseason game and started the team's second game. He has completed 10 of 16 passes this preseason for 128 yards.
Allen led the Bills for the second half of the first preseason game and played mostly in the second quarter of Friday's game. He has completed 18 of 32 passes for 176 yards and two touchdowns.