With only one healthy quarterback on the roster and unsure about the availability of starter Tarvaris Jackson for Sunday's game, the Minnesota Vikings signed veteran free agent Koy Detmer to a one-year contract on Tuesday. Detmer, 34, played his entire 10-year career in Philadelphia, where Minnesota coach Brad Childress was the onetime offensive coordinator. The offensive system used by the Vikings is similar to the one in which Detmer played with the Eagles. Jackson suffered a concussion in Sunday's victory over the San Diego Chargers, and his status for this week's game at Green Bay is uncertain. His primary backup Kelly Holcomb, who has started three games because of previous injuries to Jackson, is himself recovering from a neck injury. If neither Jackson nor Holcomb can play against the Packers, five-year veteran Brooks Bollinger would get his first start of the season. Detmer, who should be able to pick up the offense quickly, would probably then serve as the backup.