From http://espn.go.com/blog/green-bay-pa...ackers-preview

Goessling: It still has a ways to go. Zimmer was talking on Tuesday about the defensive line, which has typically had the freedom to chase the quarterback, no matter the cost. Well, Zimmer wants his defensive linemen to rush as a team, not overpursuing for individual sacks while opening up a lane for the quarterback to escape, and he saw too much of that Sunday. The Vikings also need to get better depth in their secondary, and I still think they will wind up looking for a more effective counterpart for safety Harrison Smith then they have on the roster. The linebackers have some promise, especially with what Anthony Barr can do, but the Vikings have still been too easy to target in pass coverage. I think it's probably going to take another draft, and another year of Zimmer working with players on his system, but the D is definitely headed in the right direction. You already see flashes of how the Vikings could have a really good defense in the future, and Barr and Smith could be stars.
It's why they let Jarred Allen go.