The New England Patriots have the worst defense in the league in yards allowed, points allowed, and DVOA and the eye test generally supports those rankings. One big problem (of a handful) for the Patriots defense has been the vulnerability of the linebackers in coverage.
Now not every pass completion is the fault of the linebackers- very few times are Kyle Van Noy or Elandon Roberts or Dont’a Hightower actually targeted in coverage- but they play an important role in clogging the middle of the field and taking away passing lanes for the other team.