I think LB would be a NIGHTMARE starting spot for Bale himself. He is not fully match fit and probably hasn't played LB in well over a year (or more?). So, that opens up the possibility that he makes 2 mental slips or gets caught in possession up the pitch allowing attackers to get in behind him, and his debut is remembered for shipping 2 goals with schoolboy defending. €100m for this guy?? Ouch. White hankie time.
In general though, it's been said for some time that if Bale were a continental player LB would be his natural starting position, so I can see it happening. There's also a case, as metalgear says, that having the whole pitch in front of him to run into makes him more scary than just half of it.
I hope he does well either way.