When I look at the Chicago Bears 2013 defense I see three major problems.
1) The front four isn't generating a pass rush.
2) They are missing a lot of tackles.
3) Injuries have robbed the unit of some talent.
There are other issues for sure, but if those first two problems were fixed, then all the other minor problems would go away. I'd venture that if the first two problems were fixed, that the injuries wouldn't even be that big a deal either.