The most important thing Ten Hag needs to do is develop a playing style and philosophy that suits the squad. He has done this at Ajax to great effect and needs to do this at Old Trafford as well.
The current squad doesn’t have any particular playing style and changes formations and approach depending on the opponents. That’s been a huge reason for their inconsistency.
Ten Hag needs to convince the fans, the board and even the players that this is a long-term project and it will take some time for his vision to come to life. It’s the same as what Jurgen Klopp or Pep Guardiola have done at their respective clubs and similar to what Xavi is doing at Barcelona now.
Short fixes haven’t worked at Manchester United before and solid long-term projects do attract good players.
To be very honest, a lot of the current Manchester United players do not seem to be ready to be playing for the Red Devils.
A squad revamp in several positions is needed and a mixture of the academy and transfer signings should help to rebuild the club again. Sir Alex did the same several decades back and it might be time to repeat the trick.