No, it is not offensive. It is superfluous. I would only be offended if the NFL REQUIRED everyone attending or playing to do the same thing (make the sign of the cross or pray the same prayer or wear a certain symbol).
An act of worship is not required for the game of football, certainly. I'm sure some people think that god is on their side or something, but this requires of god to be against the "other" side! So, to me it is funny to watch people perform "acts of worship" to try and gain a "win".