Curious as to when this changed because I remember Brian Bozworth in a tiff with the league over him wearing #44 which is what he wore in college. He was relegated to #55 which is probably what he blames his sucking on.
LB's haven't been allowed to wear 40's until relatively recently - like in the last decade. Used to be only 50's & 90's. Boz got some kind of exemption, or paid the fine every week, or something like that - definitely wasn't allowed back in his (brief) NFL days.
A quick Google search turned up this article.. very interesting. If he was able to prevail in court, I'm shocked that other players since haven't demanded to wear whatever number they choose. With case law setting the precedent, shouldn't even require going back to court...
https://goo.gl/sSDzIz
I remember when Reggie Bush came out of USC, he wanted the NFL to let him wear #5 as he had in college, and they said "no dice". Hmm.