I fear the back problem could mean that Andy never fulfils his potential on clay. It would be only natural if he always now approaches clay with some tentativeness and anxiety that he could aggravate the underlying problem.
I do wonder if it might undermine any confidence he was beginning to build, and as you say, he may well be anxious of aggravating an underlying problem, which could hold him back.
I was just thinking that if the back problem is something that can be sorted, something Andy is able, at some point, to put behind him, might we see Andy's real ability on clay.
I just think 2011 showed how good Andy could be on the surface (I still don't think his draws could have been that kind in three tournaments, especially on clay). He pushed Nadal and Djokovic. Then, at the end of that year he injured himself, and since then, if anything, he has gone slightly backwards on the surface. To my mind that could indicate that it's the injury holding him back, not an inability to play on the surface.