Indications had been that Barkley was likely to start for Buffalo, given that Allen was unlikely to return and Nathan Peterman has struggled mightily.
Barkley, 27, is a former fourth-round pick by the Eagles of the 2013 NFL Draft. He spent two years with the Eagles before he was traded to the Cardinals in return for a seventh-round pick at the start of 2015 season.
Barkley had brief stints with the Bears and 40ers before signing a two-year, $3.35 million contract with the Bengals back in March. Unfortunately, Cincinnati placed him on injured reserve coming out of the preseason and later waived him with an injury settlement.
The Bills signed Barkley to a contract a few weeks ago after Derek Anderson got hurt.
In 2016, Barkley threw for 1,611 while completing 59.7 percent of his passes to go along with eight touchdowns and 14 interceptions.