-No need to touch player stats or costs: catcher remains catcher-like, i.e. faster, weaker, less armored than blitzers; players are no longer over-priced as they gain another skill, but not a power skill. (Sort of like Thick Skull on some players, give it to them for flavor or when giving a "real skill" would be too much of a boost)
-No one ever picks Pro (maybe a second double on a Big Guy), this way it would see some light of day on the pitch.
-Shouldn't be too powerful as Humans already have lots of skill rerolls, but good for rerolling pushes or dauntless rolls, the little things.
-If Blood Bowl were a movie, Humans would be the good guys. They would be down on the scorecard when the time is running out (i.e. it's the second half and you're out of rerolls) when the douchebag ball-hog star player Blitzer of the team suddenly would have a change of heart and he'd decide to throw a desperate pass to our lowly lineman hero of the movie for the winning touchdown* (i.e. humans still suck and you'd be losing but you might get lucky once in a while and Pro would save your ass)
Of course these movie type scenarios happen all the time in BB but Pro would make it a little bit more likely for Nuffle to smile on a human team.
Yeah I know, "Pro" sounds stupid as a starting skill but just rename it to "Lucky SOB" and you're all set

*movie logic let's the team go from losing to winning with 1 TD