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.
The important thing for now, Ruben Amorim has started his first training at Man United, as well as his staff who will soon follow. And this is a common thing when a coach has his own staff who understand his character and strategy more easily. 
Source: First training session today for Rúben Amorim
Man United have decided this and they have appointed Ruben Amorim as their head coach and given him the freedom to choose his staff. and his contract runs until 30 June 2027. our hope is that Man United will improve, much better than they have experienced so far, especially when Ten Hag was still in office,