No whammies, no whammies, no whammies...
In other words, you would be pressing your luck.
It's worse than you think. The NFL is notorious for taking action to protect their IP. They have even sent C&D letters even when they don't own the IP being used.
This type of stuff sits in the grey area. Maybe they get caught, maybe they don't. Maybe they can win in court, maybe they can't.
The best thing to do in these situations is ask yourself how much money can you reasonably expect to make. If that amount is worth the potential risk of legal fees and time spent in court, then go for it. If it's a modest amount, then don't even bother.
Besides, won't most people rather spend on official Broncos merchandise than whatever generic design you come up with?