if you wanted to make the argument that the pats defense is old, thus it wore down at the season went along, and that you don't think they did enough to shore up this age issue and will have a step back from last season... yes, i may agree with you.
but to call them weak is, well, weak. they had a very good defense last year, certainly one of the 10 best in the league, and will likely be similar this year, although i would not be stunned if they fell back to the middle of the pack defensively this season... which would make them "average," certainly not "weak."