He has played in midfield 3 for Real (his best season) and Argentina.
Unlike other wingers, he can actually defend, so can take over from Mandzu, and obvs can offer a lot more than Mandzu in attack, which we lack.
It could be that we'll sell Dybala and play 433 next season, but it doesn't have much to do with Di Maria exclusively IMO. You should be alarmed with the Martial rumors too, more alarmed in fact, if you're worried about Dybala leaving.