It is common for a coach to bring his own staff when joining a new club, Ruud van Nistelrooy could work with Ten Hag again at the new club later. I think almost all managers would do the same, there is nothing surprising in this. But Manchester United fans should thank Ruud van Nistelrooy, because he managed to boost the team performance even though his period was relatively short.
Ruben Amorim and his staff have been effectively managing Manchester United since November 11, and will make his first debut after the international break against Ipswich Town. Ruben Amorim first three games with Manchester United were relatively easy, it was a good momentum to gain the fans trust and start a new era. We need to be patient to see what will happen in the first three matches first.
That is indeed common and I think they made the right choice. The Primeira Liga is of course not the best in the world, but he has still been competing with teams like Porto and he won several titles despite being very young. For many people this might look like exchanging one young coach (van Nistelrooy) for another young coach (Amorim), but checking his history, there is some considerable success. The Premier League is a different calibre and he will quickly find out, but he was doing a great job for Sporting.
Though looking at the facts, €10 million isn't that much to buy out a coach in the middle of the season while the team, Sporting, has a perfect record of 33/33 points and 39:5 goals... I mean this is quite a good deal still as €10 million is close to nothing these days. Bayern Munich paid €25 million for Nagelsmann back in the days and that was not in the middle of the season.