This old-fashioned system might well be seen as rather out-dated these days, but with the right players on the field, and when deployed in the correct manner, there is no doubt that it can be just as effective as it ever was.
For Manchester United to operate to their maximum potential in a 4-4-2, some subtle shifts would probably have to be made by Mourinho, both in terms of personnel on the pitch and the tactics which he hands down to Pogba and his team-mates.
For example, although the French powerhouse is an absolute physical monstrosity, it may be required for Morgan Schneiderlin to come in alongside him, simply to add an extra level of stamina and manoeuvrability compared to former Bayern veteran Bastian Schweinsteiger. The United youth product hardly lacks in these areas, but he would nevertheless require support in the central area of the pitch in order to overcome what will often be a trio of opponents in that area.
From the position of being in the heart of a midfield duo, Paul Pogba would have an increased degree of responsibility in comparison to his current club Juventus and indeed on the international stage too. Nevertheless, he would surely be the fulcrum to the Manchester United side in their build-up play and offensive efforts too.