You may receive a "complaint" in the form of a cease-and-desist, or a lawsuit. Do you really want to take that risk? If you are hesitating it is probably because the logos make the app better--but that's the point! There is value in the logos and the right to benefit from that value is 100% the NFL's.

The app is awesome, though. I hope you're able to keep it going. I'd get rid of the logos now, though!