Bama's defense wasn't the problem. They held Auburn to 28 points until the miracle play happened, which was one of their lowest scoring games of the year. Say what you want about 296 rushing yards, Bama stuffed Auburn for much of the game.
Bama's real problem this past year was they got away from the power run game. They lost some of their physical mentality on the offensive line and were forced to throw a lot which isn't a good thing for any team. They got away from what's made them successful the last 7 years and that's why the OC tucked tail to Michigan.