Funny you can come to the conclusion that he can "start left back without much difficulty" and yet you've probably never seen him play left back!
We all know exactly what Gareth Bale plays like, I'm sure there's not a member on here that hasn't seen him play at least 10 times, many more in most cases. And general consensus seems to be he could be exciting as an attacking left back, so could Stewart Downing for the record, but is quite a long way off the standard needed to play in his actual position for them.