#3 Manchester United's static midfield
Any effective midfield has a player who's dynamic enough to receive the ball in tight spaces and progress it either by quick passing or dribbling it forward. Neither Fred, nor Nemanja Matic played that role for Manchester United against AZ. The presence of Mata in the team suggested that he may have played closer to this midfield duo and provide that spark, but the Spaniard played closer to the striker rather than dropping deep.
When Fred arrived at Manchester United, he showed glimpses of such dynamism, but against Alkmaar, he was limited to passing down the sides and didn't really show any signs of carrying the ball forward.
The only real midfielder that Manchester United have who's shown that he's able to link play with the attackers effectively is Paul Pogba but his current unavailability poses a big question of this squad.