#1 Paul Pogba
Manchester United's record signing and the most expensive transfer (at the time), Paul Pogba had a largely disappointing return to Old Trafford. The world's costliest player tag sort of seemed to weigh him down and he could just manage 9 goals and 6 assists in 51 games.
This season, however, after Neymar's record-breaking transfer to PSG and the arrival of Nemanja Matic for the midfield anchor role, has somewhat unleashed his full powers. He no longer had the tag of the most expensive player and was relieved of his defensive duties. This enabled him to play a much more free role on the pitch and he's already racked up 3 goals and 6 assists in 18 games this season.
Those numbers don't really show the full picture. Pogba's absence due to injury coincided with United's poor form. The stats are there for all to see- with Pogba, United have claimed an average of 2.6 points and scored 3.2 goals/game whereas in his absence those figures dropped to 1.8 pts and 1.5 goals respectively. This just proves how much United depend on their enigmatic midfield general.