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.
The Red Devils decision to make Amorim the new coach was the right move given his coaching ability, he has shown great ability at Sporting and many hope he can bring better and innovative tactical approaches and changes to Manchester United later, but the sacking of RVN is something controversial in my opinion after what he did as their interim manager, RVN is a legend and he knows a lot about how to coach a team so manchester united should have considered giving him an assistant coaching position or become a youth team coach instead of sacking him, RVN was previously contracted as ten hag staff and actually he has a long-term contract with manchester united but I think the arrival of amorim has changed RVN fate at this time and made him unemployed
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Sometimes it is not about who's brought in to manage the team that matters but the system that is being operated at the club could be a determining factor as to what later plays out, most of the coaches that Manchester United have hired in last 10 years have either succeeded somewhere else before they were hired at Manchester united but tend to underperform at Manchester United, so I think if they really want the new manager to succeed then they have to create a conducive atmosphere for him to operate and also ensure that the club's hierarchy doesn't interfere with the dressing room business.