You are correct, defensive injuries hurt, but it was more about offensive playcalling. Any team can put up yards, so #6 means zero to me when you sport a losing record.
So if you have a bad defense that allows other teams to stay in the game, and win, that's on the offensive coordinator?
Look. If it wasn't for Tosh and what happened at USC we'd be looking at Tua playing on the west coast. Specifically, for Sark. There's a close relationship. There's always been one.
Now, if you like, take a look at what he did with Matt Ryan this season and then weigh it out with "he didn't do a good job."
Ryan didn't break any of his own records this season but did put up numbers close to those. It was the 2nd highest totals in QBR, Yards per attempt and second highest yards per game of his career. His completion percentage? 2nd highest again. TD's? 2nd, again. It was the lowest INT rate of his career. (H/T to Teddy on compiling those figures and he's a basketball guy.)
If wins and losses are the end all, be all, then Kiffin wouldn't have had the success he had at Bama. Neither would Locks. Hell, if we're going to use the NFL to grade how a guy will do in college Saban and Stallings wouldn't have had the careers at Bama they did.