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1. The entire game plan against Denver
2. Inability to fix the run defense or create consistent pressure
The Steelers faced a ton of injuries on the defensive side of the ball and played almost an entire season without getting their four starting linebackers on the field at the same time.
Still, there were some problems for LeBeau, who's possibly the greatest defensive coordinator of all time.
LeBeau's unit couldn't create consistent pressure on quarterbacks. That's a product of the injuries. They couldn't stop the run consistently either, a major contributing factor in losses to Baltimore and Houston.
Against the Denver Broncos, LeBeau committed to making Tim Tebow beat the Steelers through the air and believed that it couldn't be done. Unfortunately, after a quarter the Broncos adjusted to the heavy fronts Pittsburgh was employing and uncorked Tebow's left arm. The next thing Pittsburgh knew they were playing catch up.
The adage from a lot of fans I've spoken to is that the defense never played a complete game all season. That's pretty accurate, although they did bail out the offense regularly down the stretch and notably against the Chiefs.
It's hard to blame someone with LeBeau's resume and with the respect he commands (and gets from this writer), but the defense ranked first overall yet couldn't close out opponents in big games.