From 1967 to 1999, 22 of 33 defending Super Bowl champions started a future Hall of Famer at quarterback in Week 1. Since then, only one quarterback for a defending champion is even eligible for the Hall of Fame right now (Kurt Warner), but 12 of the past 16 defending champions started a QB who will likely end up in Canton (Tom Brady, Drew Brees, Eli Manning, Peyton Manning, Aaron Rodgers, Ben Roethlisberger and Warner).
So it's rare for a defending champion to have a questionable quarterback situation going into the season.