You got players from the Moyes and Van Gaal era, who each came into the club to play under their respective manager's system.
Then Mourinho went out and brought few of his own big name signings, again they were meant to play under his system whilst trying to blend in the players from the previous managers.
What you essentially have ended up with is a horrible situation where the entire squad of players, are unable to really get to grips with wtf is going on at the best of times.
Do not forget there are still one or two players from the Fergie era also.
Now, how or why United's best player Paul Pogba didnt play in the Manchester derby is a total and utter mindfeck.
He is by a long way, their best player and if deployed in the right kind of system can be imense. Same could be said of Lukaku, who imo is a world class player well capably of scoring 20+ goals in the league alone, each season ( requires the right system and set up and selection for him to do it ).
So ask yourselves one thing, after nearly a third of the way into the 2018-19 EPL campaign, how have Manchester United found themselves on negative goal difference and miles away from Man City and Liverpool ?
Answer is simple - Jose Mourinho has tried his utmost best to create a side from players that were Van Gaal's recruits and before this, Moyes's recruits.
The side simply hasn't got a fecking clue how to match Liverpool or City in terms of flair, confidence and style of play.