Amorim needs time to get the current Manchester United squad to be prepared for the rest of the season. We shouldn't use the impact that Ruud got to expect same thing from Amorim. The players knew that their position weren't guaranteed with Ruud therefore they played very well to try and get more playing time but under the new permanent manager, after some players get it that they're part of the first team, they won't be overplaying again but to reserve their strength for other games therefore we mightn't be getting the same energy we use to get from them and it can affect the teams performance therefore it depends on how the manager handle the situation.
If my memory serves me right, I doubt Man UTD, in their current situation usually give new coaches time to adjust and fit into the club and I don't expect Ruben Amorim's case to be different. They saw how he helped his former club defeat Man City in the champions league and would be full of expectations for him to be able to move United back to the top four by winning games and since Amorim has been been heard saying at his unveiling that he'd do so many things with the club, I just hope he can be able to deliver and not join the long list of coaches who although have potential, can't be really show their talent just because the club's impatient.