The winger got a chance to start for United on the right-wing and delivered a decisive performance.
Diallo scored twice, once with his head and once from outside the box, in a 2-0 win over PAOK in the Europa League on Thursday.
"He made a difference for us today because of his goals," van Nistelrooy told TNT Sports.
"But he was very sharp. I think, in the first half, he was one of the few who was at his level. Well done to him, good game.
"His first goal was a good goal and a good Bruno assist. We like when we were in those spaces, where Bruno was to put the ball in between the keeper and the backline - you can see the three players in the movement, it’s something we’ve worked on a lot since the beginning of the season.
"They are always dangerous areas to cross from and we needed this. Amad made his own goal (for the second goal), Christian (Eriksen) tried to cross it and then he came inside on his left and getting it in the far post was excellent. He won his own duel and took the shot for the goal - a good goal. He has been incredible actually in training, he was never affected by not playing. He always trains well and is asking me to do more. That’s an example.”