For me, Van Nistelrooy was doing very better at old Trafford than Amorim, yes I know his performance at Leicester isn't that much impressive reason being that Leicester has more midfielders and fewer attackers than Manchester United and Van Nistelrooy was better at attacking football which is the reason he performed very well in his first four matches at old Trafford. The only forward at Leicester is Vardy who isn't enough tho provide his magic up front. I believe with more strikers, he'll do better. My only fear is if he would make it past the relegation zone this season.
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.