I'm honestly more prone to blame the coaches, even if it's a talent failure.
Rivera is just not a good talent evaluator. He puts entirely too much faith in guys like Byron Bell and Captain Munnerlyn. Much as I love guys who are all hard work and give 110% on every play, sometimes they're just not good enough.
Rivera isn't in charge of talent evaluation, but rather is in a position (and has been since he has gotten here) to make something outof nothing. Hopefully he is allowed to eventually pick guys on the open market he actually wants when the cap is fixed. But he has shown that he can at the very least build a top 5 defense anywhere he goes. That's talented in my book.