Hmm...well, let's see who they have played against so far this year...
SEA - 7th ranked pass defense (in yardage allowed).
CAR - 23rd ranked pass defense
STL - 27th ranked pass defense.
NYG - 31st ranked pass defense.
SF - 32nd ranked pass defense.
That's some top-notch competition there. If you think about it, as long as whoever the QB was could get the ball to the receivers, they were going to do well. Their division isn't the toughest in the world either. The fact that they don't have a whole lot of TD's to their credit is a little scary considering the caliber of defenses they have played.
It will get harder from here, but not for a while. Their toughest matchups will probably be Week 10 (DET), Week 12 (JAX), Week 14 (WAS), and Week 16 (PHI). Boldin/Fitz could very well be pretty good for most of the season, but you might want to temper your expectations for the weeks listed above.
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