Signing Ruben Amorim as the new head coach of Manchester United sr team was the right decision they made, it came late because they waited for Ten Hag to cause most of this damage for the team, because if they signed him earlier than now, the team might have been contesting for top four by now; but it is not too late for Manchester United because with the qualities Amorim have as a coach based on what we have seen in his last team, he is capable of bringing glory back to Manchester United.
Amorim moves from Sporting looks like a pre contract that was signed before now else I'm not sure if the way Sporting are having good times defeating other teams will sell Amorim in this die minute. My assumption is that he was in contact with Manchester United before now probably from June but have ETH sometimea for redemption but it was going hay and they have to call him to order to call back Amorim to take over and since Manchester United is everyone's dream team, he took over the job immediately.
On the departure of Ruud Van Nistelrooy, I think it was because Amorim wanted to work with his complete coaching staff, and Manchester United will not want to risk anything so they will not be responsible if the new coach fails to make the team great again. Sin Ruud Van Nistelrooy is a legend of Manchester United, I think it will not be a problem and he can be called when his help is needed in the team anytime.
I'm sure that coaching staffs was part of the documented contract otherwise he dare not bring another person when he is the person they needed for the job but fair enough. If the coaching staffs are part of his strategy he has been using to win teams so far, they have no choice than to allow him and Ruud was just a caretaker meant to help put the players in good organization which he has done, he was paid and time to move on to the next big club.