Midfielders
Paul Pogba

The towering Frenchman has been impressive this season. The pressure to perform is always going to follow him whenever he plays. He has scored an impressive four goals this season and there is the feeling that more is to come.
He seems to be enjoying the best spell of his United career so far. Mourinho's decision to play a three-man midfield seems to have paid off. Pogba seems to have more freedom to open up play and score goals.
Ander Herrera

The Spaniard was in contention for a place against Young Boys but did not play. He is versatile and can fit right in a three-man midfield. He can play in any of the three positions and offer cover whenever Pogba is on the offensive end.
Herrera has the necessary experience to read the game situations and do the 'dirty work' when needed. Additionally, he will be crucial in halting the counter attacks and looking for spaces for the forward runners.
Marouane Fellaini

The big Belgian may be used once again in an unfamiliar deep midfield role against Watford. He basically offered protection to the central defenders to Mourinho's liking. He will once again be tasked to do the same given that Fred has not yet settled into the team.
His height and threat on set pieces will be crucial as seen in recent matches. He may not be technically gifted but winning aerial duels and being a target man up front will be important for the team.