This skill is a Special Rule that is only available to specific Star Players. Please visit this page to find a list of current and past Blood Bowl Star Players.
Old Pro
One per game, Helmut may use his Pro skill to re-roll a single dice rolled as part of an Armour roll.
Chainsaw players don’t usually live a long life on the pitch, either by getting injured themselves or being sent off. Typically you are going to be targeting one of the better players on the opposing team, so I would probably look to use it at the first opportunity where you don’t break armour. Assuming you haven’t already used the Pro skill that turn as you can’t use it twice.
If the Pro roll fails, is Old Pro used up, or does he get another chance at it later in the drive? In other words, is Old Pro used up by the attempt, or only if the armour die is actually re-rolled?
I can see that being argued both ways. I think that’s one you will need to email Games Workshop, the email address for rules questions can be found in the FAQ / Errata document they release twice a year. If you do get a response then please let us know (they’ve never replied to any I’ve sent though). Personally speaking if I was your league commissioner, then I’d rule that if you failed the Pro roll, then you can try again later on until you actually get to reroll one of the Armour roll dice.
Only 3 years later, this was finally addressed by GW in an FAQ:
“Q: When using Helmut Wulf’s Old Pro special rule, do you still
need to roll for the Pro skill? (p.131)
A: Yes. Old Pro offers an alternative way to use the Pro skill,
so you must still roll a D6 as described in the skill. If the Pro
skill roll fails, then you may attempt to use Old Pro again in
a later turn – the once per game use comes into effect only
when you actually re-roll a dice as part of an Armour roll.”