3 season is enough for Ten Hag so i think when Manchester United has decide to sack him it is the right decision because they didn't rush and give almost 3 years for Ten Hag to build solid squad but in fact Ten Hag has been failed although at that time he has spend a lot of money to bought the players and about Amorim if i am not mistaken his position in Manchester United is the coach not as the manager so unlike Ten Hag, Amorim was not given full authority to decide which players to buy 

Amorim position is similar just like Enzo Maresca in Chelsea that about the decision to bought the players Maresca have no full authority for the transfer and regarding big funds i am sure Manchester United will provide big funds to Amorim however recently i have read news update that Amorim will bring all of his staffs in Sporting because the staffs is really important for Amorim so i think probably Amorim will used his own assistant then most likely Nistelrooy will be leaving too