Manchester United have good players,what they lack at the moment to me is a good coach.A good coach is someone who can come to the team,and motivate those boys at the club,inject into them a winning mentality,and always encourage them whenever they make mistakes.Coach don't always criticize players,else,they will fumble and play rubbish on the field.When players are not inform,it is the work of the coach to build confidence back into those players so they can perform well.Take a look at how Arteta revive every dead thing in Arsenal,and they are now competiting fiercely for the tittle.For two seasons now,they've been the only team challenging City for the tittle.
Encouragement and believing in the ability of that player is something most times only a coach can do better than anyone else, if a coach is making a player believe he's good, other people's opinion may not matter much because the coach is the first and most important critics to the player but whenever the coach is doing the opposite of encouraging the players, their performance is definitely not going to make sense.
Arteta and guardiola, even klop are good examples of how much a coach can impact a team, Arsenal might not have won a trophy in the last few seasons, but their present form is a threat to many teams both in the English league and the other leagues, I believe that if Manchester united gets a good coach who can go beyond just criticizing and quarrelling the players, they are going to so much better, Rashford needs a coach who's going to make him understand how much he can achieve if he puts in more effort to bringing out his best and so are many other Manchester united players.
Any team that really wants to improve and make a whole lot of progress, the ability of the team lays mostly in the ability of their head coach because they will be playing according to his instructions and tactics, so I'd the coach is giving Instructions that may not help the players, then most definitely they are not going to perform as much as they should have done if they had a better coach who has been encouraging them well enough.