Cooper Rush is in line to be the Dallas Cowboys’ starting quarterback for as long as Dak Prescott remains out. The team confirmed Saturday that it signed the 28-year-old and elevated Will Grier to the 53-man roster. Rush will serve as the Cowboys’ starter, whereas Grier will be the backup. The move was expected as Rush practiced this week, taking first-team reps. “He’s as rehearsed in this offense as anybody,” head coach Mike McCarthy said this week. During last Sunday night’s loss to the Tampa Bay Buccaneers, Prescott suffered a thumb injury that required surgery. He is reportedly expec…