ESPN's Chris Mortensen reported that the Bills (0-1) will bench Nathan Peterman and tab Allen as the starter later in the day.
Allen, who was the seventh overall pick in April's draft, will become the second member of the Class of 2018 to start at quarterback this season. Sam Darnold, who was the third overall pick of the 2018 NFL Draft, was under center in the New York Jets' 48-17 season-opening victory versus the Detroit Lions on Monday.
The 6-foot-5, 237-pound Allen drove the Bills to their only score of Sunday's 47-3 setback to the Baltimore Ravens -- a field goal by Stephen Hauschka. He also had a touchdown pass that was dropped by wide receiver Kelvin Benjamin.
Allen finished 6-of-15 for 74 yards. Like Peterman, he absorbed three sacks and was flushed out of the pocket on numerous occasions, leading to his 26 rushing yards on five scrambles.
The Bills managed just 10 first downs and 153 total yards, were 2-of-15 on third down, and averaged just 2.5 yards per play.
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.