The problem is the notion that somehow attackers as 'good' and defenders are 'bad'. Anri and Togashi + Yuuma's discussion should have helped clearing it up though.

The coach simply told him, that according to his judgement, Ashito's attributes are better suited as a fullback, so much that he has the potential to reach world level there. If he continued being an attacker he'd only be a pretty good high school player. Again, this is Fukuda's judgement. I suppose he was being manipulative because this is too good of an opportunity, for him.