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While Jay Cutler put up a lot more yards (over 4500) than Cassel, he also had 18 interceptions, a lower QB rating, and a worse completion percentage. Plus he has never had a playoff-calibre season as a starter (Denver has finished 8-8 and 7-9 the past 2 years, ending up with the 12th pick both years).
Denver may have the worst defense in the NFL. Cutler is not the problem. Hes great.