Last year's defensive rankings compared with this year's:
Pts Allowed
2014: #8 (19.6)
2015: #1 (18.2)
Yds Allowed
2014: #13 (344.1)
2015: #15 (341.3)
Pass Yds Allowed
2014: #17 (240)
2015: #21 (253)
Passer Rating Allowed
2014: #10 (84.0)
2015: #9 (83.9)
Rushing Yds Allowed
2014: #9 (104.3)
2015: #2 (88.8)
Rushing YPC Allowed
2014: #9 (4.0)
2015: #11 (3.9)
Turnovers Per Game Generated
2014: #13 (1.6)
2015: #16 (1.4)
What's interesting is that the Patriots' defense has improved over last year's unit, not only in terms of rankings, but in actual production as well (points allowed, yards allowed, etc.). And they've done it even though they're generating slightly fewer turnovers a game. That is, they're not relying on the turnover as much to stop people - they're just getting more stops. Even in third down defense they've improved slightly.
2014: 40.2%
2015: 39.6%
This may not be the *best* defense in the league, but it's clearly one of the better units in football. Certainly good enough to carry the team to a championship, even with the injuries on offense.