The Patriots have historically struggled when facing defensive lines with elite interior pass rushers. When Tom Brady is contained in the pocket by the defensive ends, the defensive tackles are able to crash inside and force Brady to make an off-balance throw. It's been the perfect recipe and it's held true until this season when Brady added a new move to his tool box.
Brady's ability to slide outside of the pocket- a small motion, but a new one- to buy just a second or two of more time has helped the Patriots offense compensate for the weak interior line play. That said, the Patriots haven't faced an interior this with Detroit's talent (although the Bills and Jets can compete). This is a very big challenge to see how much Brady's mobility can adjust against the best 4-3 defensive tackle in the league- Ndamukong Suh.
The Lions have rode their elite defensive line to a #1 ranked defense (which is a familiar story for the Bengals, Bills, and Broncos) while their offense sputtered without a healthy Calvin Johnson and Reggie Bush. Both are expected to play against the Patriots.
Detroit is currently allowing a league best 1.39 points per drive, ahead of the Cardinals (1.46 PAPD) and Bills (1.53). This would rank as the 66th (of 509) best mark since 1999, or rank them just inside the top 13%. As a point of comparison, this matches the 2013 Panthers (1.39), but ranks behind the 2012 and 2013 Seahawks (1.38 and 1.22, respectively). [The Patriots rank 14th with 1.85 PAPD]
It's a good defense, but it's not earth shattering.
Still, it's not fair to rank the defense against those that played through different defensive rules. When adjusting for seasonal differences, the Lions defenses jumps to 43rd best since 1999- but falls behind when compared to contemporaries. The 2013 Panthers move to 27th, while the Lions compare better to the 2013 49ers, the fourth best defense on the year.
While the Lions defense might not be a historical unit, they're still extremely good. The Patriots will need to make sure the offensive line is up to the task of stopping, or even just slowing, the Lions fantastic defensive front.