Backup quarterbacks pressed into duty due to injuries have won Super Bowls more often than you'd think.
Jim Plunkett was a failed Heisman Trophy winner backing up Dan Pastorini in 1980 for the Oakland Raiders and had thrown one pass all season when Pastorini broke his leg in the Raiders' fifth game. Plunkett took over and the Raiders won a Super Bowl a few months later. Jeff Hostetler had started two NFL games when the 1990 New York Giants needed him to replace an injured Phil Simms late in the regular season. Hostetler played pretty well and the Giants won Super Bowl XXV.