#2 Tactics
Mourinho's tactics have been heavily criticized of late, and been termed as overly defensive. Even on home turf, United have at times this season been overplayed by smaller teams.
Pogba is the type of player that loves to get forward and create chances. That is where he has been most effective, even at Juventus.
In a league that is considered highly defensive, Juventus pumped in the goals and played with attacking flair. Pogba played as a typical central attacking midfielder or left central midfielder. And he had other players around him as cover, which reduced his defensive responsibilities.
But Mourinho's defense-first tactics could drive Pogba out of United. No player wants to be forced to play in a position they are not used to, especially when there are better teams ready to accommodate them.