Coach Sean McDermott announced that Allen (elbow) has been ruled out for Sunday's game against the Colts and is considered week-to-week, Chris Brown of the Bills' official site reports.
McDermott also relayed that veteran Derek Anderson would take over as the team's starting quarterback in place of Allen, who has been diagnosed with a right elbow sprain but won't require surgery at this time. Allen likely will be withheld from practice this week and treat the injury through rest and rehab, with the hope of returning to action after missing only two or three games. Nathan Peterman will serve as Anderson's backup while the rookie is sidelined.