Lets assume a player is faced with the following hypothetical situation:
If the defender does nothing the offensive player will run by the defender and score a TD or if the defender does X the player will not score a TD but there is a 1% chance the defender offensive will be paralyzed. Should the defender do X?
Impossible to answer as you posed it.
Is there anything BESIDES X that the defender can do to try to prevent the score? Just because he can't do X doesn't mean he has to do nothing. If what you're getting at is that UNLESS he does X he's going to score, then I would ask....
Is X against the rules? If it is then obviously he shouldn't do it. If it's not, I don't see why he wouldn't do it. The chance of injury is unimportant if it's not against the rules.
Care to clarify the hypothetical so we have a better idea of what you're asking?
If what your getting at is if something is against the rules because of safety concerns, but it's the only way to make the play, should the defensive player do it anyway?
My answer to that hypothetical is no. Winning at any cost is NOT the way to play. Now if the rule is not there for safety, then I say go ahead and do it. For example DB's are taught if that they are falling down and it's gonna mean a sure TD for the WR, go ahead and pull the WR down and get a pass interference call. That's better than giving up a TD. I'm all for that.