Manchester United asked Van Nistelrooy to be an interim for a few games. It's until they were finalized a deal for a new coach, Amorim. He has a contract with United just for 4 games, then he stepped down as an interim because his contract ended.
There's no need to fire him because how long he coached the club has been mentioned on his contract.
It seems like it's unfair because he managed to bring about change with Man United. This condition does not need to be a big problem, Nistelrooy has understood this situation. He has understood the situation at hand and it is a risk. Nistelrooy has shown that he is a coach who has good potential. So there will be a lot of opportunities that come for him because he is a modern football coach. Actually, I can't wait for Amorim's debut with Man United. I hope his arrival brings better changes than Nistelrooy and Ten Hag.
If I was also in Van Nistelrooy’s position, I would have moved on and not allow anything to keep me Bach when I know my worth and know what I can achieve while working as a boss rather than working under someone. I just hope he gets a good club soon and start his managerial role fully as an head coach. I see he has a lot of potentials which he can use to transform a team to become successful. If not that Amorim is already in the coaching frontline before him, they wouldn’t have gone for him and just permanent Van Nistelrooy which may even save cost for them. Amorim coming to Manchester United has been full with a lot of positivity and enthusiasm for them to do well in the remaining of the season, let’s see how well he can transform this team within the first few games he’ll be managing for them.