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.