McCarron returned to practice for the first time since sustaining what as initially reported to be a hairline fracture of his collarbone in a preseason game versus the Cleveland Browns on Friday. Bills coach Sean McDermott has since dismissed that report, but admitted that the quarterback is dealing with soreness.
"He does have some shoulder soreness," McDermott said. "We'll see how he progresses through the day."
"All part of the plan, part of the process in the terms of the stage of his development," McDermott said of starting Allen, who was selected with the seventh overall pick of the 2018 NFL Draft.
Allen replaced McCarron and led the Bills' first-team offense on an 80-yard touchdown drive in the second quarter against the Browns.
While playing three possessions, Allen completed 9 of 13 passes for 60 yards and a 2-yard touchdown pass to wide receiver Rod Streater.
The 6-foot-5, 237-pound Allen passed for 1,966 yards with 19 touchdowns last season at Wyoming. He ran the 40-yard dash at the NFL Scouting Combine in 4.75 seconds with a 33 1/2-inch vertical jump.
Peterman took over from Allen early in the third quarter and closed out Friday's game, completing 8 of 10 passes for 113 yards.