Outside of the injuries, the three major areas that have plagued the defense this season are poor angles , bad tackling, and gap discipline. Those are reflective of coaching. The Bears weren't this bad at it last year with Lovie. Chris Conte and Major Wright went from serviceable safeties to maybe out of the NFL in less than a year.
Even the game planning by Tucker has been horrific this season. The Bears had no idea how to defend the read option against Washington last year because Mel Tucker was telling his DE's to crash down on every play, leaving the outside WIDE open. A hobbled RGIII tore our defense apart thanks to this awful strategy.
I also don't think it's a coincidence either that our defense is so pathetic at getting to the quarterback. Mel Tucker's defenses in Jacksonville were routinely at the bottom of the league in sacks.