I think the blame should fall on whoever is making the personnel decisions around there... over the last 5 years, Detroit has given away (or allowed to leave) all of its best defensive players. Some of the best lineman in the NFL. No they couldn't stop a high school from running on them.
Also, the secondary hasn't been even decent since Dre Bly was playing on the team...and has never been great (Never).
The depth is weak, when they lose one(and they always do) there is no one there to step up. Player play the game..I honestly can't say the coach isn't part of the problem, but I haven't seen him blatantly miss call a game of gold on an important possession call.