Amorim would have wanted Ruud to stay for a while though, like for example to get a report on who looks like how. Remember, this isn't a fifa or football manager game, players do not have ratings written on top of them, nothing is for certain and if nobody tells you anything, all you can base your assumption on is previous games, but that is based on another coach, and things are different now.
So if Ruud stayed, he could have given Amorim some insider information, that way Amorim could have started a bit better. In any case, I am sure that we are going to see him do fine, he will need just a few games adjustment period, and if he can achieve that, rest will be much easier for him to get better results on. When you are a new coach, the hardest part is to get to learn more about players, and their personality, sometimes takes as much as months but after you do learn more about them, you end up not getting anything worse, you end up only growing better.
Yes it is possible that Amorim would have wanted Ruud van Nistelrooy to stay for a while but I don't think Ruud would have liked the idea to work with someone that is his junior and his supposed replacement. If Ruud van Nistelrooy had put his ego aside and decided to stay, it will give Amorim more edge to make a positive impact quickly, in terms of settling quick and bonding well with the players and establishing strong relationship too. And now that Ruud has left the club, I know Ruben Amorim will also not find it difficult to settle at the club. He can adapt and also bring out the best in his players.