I think the problem at Manchester United is that connection between the players, it is very important to build team spirit and play as a team. They also need to connect with the fans. I would suggest Erik Ten Hang changing the energy around the stadium and lifting up the team spirit just as Mikel Arteta did at Arsenal after joining that should bring a major boost to the team.
Also from the mental side, the young players at Manchester United are still too spoiled. I think Erik ten Hag should also put more emphasis on this for his players. Mostly because they are young, so to be at a giant club is mentally a lot more difficult. Competing with other clubs will greatly affect the game which is not very stable.