#3 Resolve the fullbacks' situation
Most attack-minded teams such as Manchester City and Liverpool create a lot of chances from the wings. They cross, cut inside or play low crosses.
United's fullbacks need to be ready to block these crosses and also cover up for the overlaps runs with the help of CDM.
If the fullbacks maintain their proper positioning throughout the game, this would surely freak out teams like Manchester City. United could also hit on counters as attacking teams usually play with a high defensive line.
To prove this point we can look at the example of Man City vs Arsenal game. Man City won 2-0. The first goal came from Sterling who cutback inside beating Bellerin and Guendouji and later shooting it past Cech and into the net.
The second goal came from Bernardo Silva who came as a substitute for Mahrez and played on the right wing. He was left unmarked on the right side and was given a clear shot at goal.
If fullbacks were properly positioned, both these goals could have been stopped and defeat could have been prevented.
Luke Shaw usually joins ahead in the attack with his forward runs. But he should also be able to quickly fall back if the possession is lost.
Likewise, Valencia has done well at the right back spot and should be starting games when he regains full fitness.
He should also be able to block the crosses which he usually doesn't, causing all sorts of problems inside the box.