Four games is too little to draw conclusions about how good a manager Van Nistelrooy could have been at Manchester United. It matters who their opponents were, and it was a time of change, the players knew there would be a change of manager, maybe there was some positivity in that too, because they saw that they couldn't achieve anything good with ten Hag, by the way, something similar might be happening with Amorim.
Four games was too little I agree but starting with a good performance is a plus. You could bear witness to the fact that the players showed high enthusiasm then than now. If you watch Leicester play, you'll enjoy their games more than Manchester United. The only thing that prevents can Nistelrooy from excelling there is lack of competent strikers, they miss lots of chances, and in return concede and loose. I think can Nistelrooy coaching style carries more optimism than Amorim although I'll like to mention that Amad's absence really weakened Amorim, just like Rodri's absence weakened Guardiola.
We can safely make an assumption on how he is doing over at Leicester. Are they overachieving? Are they underachieving? Or are they doing what is expected of them? At the moment, it is 99% guaranteed that they will relegate, and they fired their manager because he was doing badly, and got Ruud instead, who didn't recover the team and they are still relegating.
This means, expectations of Leicester was to relegate, and they are relegating. He isn't underachieving, but he isn't overachieving neither. This is why we have to consider him an ordinary manager. If he took that team and somehow kept them in the league? Then I would say he is a great one, but since he failed to do that, I do not see him as anything special.