Well, there's injuries, and then theres INJURIES. For the 3rd year in a row, the Pats set records for the number of different starters they fielded due to injury. (41, 42 & 44 respectively). Other teams have suffered as badly, but none ever put together a winning season, let alone won super bowls.
The Pats are all about quality depth. They went thru 5 starting RB's last year, including two guys who started less than a week after they joined the team (Zeroue and Evans).
They had six DBs on IR by November last year. Just how many do you think teams carry?
No team in the NFL has the depth the Pats do. While their FA pickups may not be flashy, they're solid (last year being the exception to the rule - Starks was B A D!). This year they've picked up Warfield (8yr starting CB), Jones (8 yr starting safety), Mitchell, ST demon & depth at LB), Caldwell, who may step up after getting out of Marty World.
With the rookies they added, this team is better than last years. It remains to be seen if they can remain healthy though. I'm beginning to think they are jinxed.
3/5ths of their OL (LT, C, & RT) were out for the season by November.