Originally Posted by
pele
I think you're right. I believe that the mentality button transfers a small percentage of ability between attacking and defending. For example, a DM with an attacking mentality might received 1% attacking bonus, but will lose 1% defensive ability (for instance, Liverpool's Fabinho). Similarly, an AMR with a defensive mentality will get a 1% defensive bonus and lose 1% attacking bonus (for instance, Chelsea's Willian).
I really want to give my fullbacks attacking mentality, but my league is so tight at the moment, I dare not take a risk. So here's hoping someone else will carry the burden of testing it out over a stretch of games.