#3 Not pressing high when not in possession
Manchester City made a staggering 643 passes on Sunday as opposed to United's 319. That was possible simply because United did not press high when not in possession and let City dictate the game to them. They were waiting to get the ball back in case of a missed pass or fumble but made no attempt to get the ball back themselves.
City, on the other hand, pressed very high and got the ball back from United whenever they lost it. They did not give United's players any time on the ball nor did they let them play within their comfort zone.
United need to learn how to press high because no top opposition is going to give the ball away easily and certainly not Pep Guardiola's Manchester City. Jose Mourinho needs to instill this in his team and make it clear that having possession does make a lot of difference to a game.