I agree that space behind Mata is a big issue and first noticed it in the DC match. DC started to exploit it and then Nyarko went out. Portland exploited it but Melano was poor.
I think, though, that it is more a tactical choice than a reflection on him. Back when we were playing 3 at the back, I was struck by how disciplined he was in terms of holding his position. I think it is a question of finding the right balance between the offense he generates as an overlapping fullback and the defensive risk.
I will, however, fully lay blame at his feet for getting beaten on the corners this week, though I haven't gone back and watched them again to see if there were any mitigating factors.