This may be very true that the players may not be giving their all to the team. Manchester United will need to sell off some players for them to be able restructure the team and pick young talents who are ready and willing to gibe their all for the growth of the team and their own growth, Amorim is a man who works with building of systems to achieving a long term goal so I believe he is the good person for the job and will need time and resources, two seasons like you mentioned will not be a bad idea at all for him to be able to pull that through.
We have already seen proof that Manchester united's squad is not capable of performing well. The bonding between the players is very weak. That's why Manchester United failed to do well even after changing coaches several times. And the united management has already realized their weaknesses. So Manchester united wants to build a new squad with the help of amorim. They are trying to sell several players. At the same time, they are also trying to add new talented players to the squad. As a result , Manchester united may not be able to improve quickly. However, in the long run, they will be able to do well.