Fernandes scored twice - his first goals for the season - in last night's 5-2 win.
Van Nistelrooy said afterwards: "You know, with his qualities, we knew he just needed some luck to get the goals going and then they will come with many. I'm delighted for him. He's leading the team through many, many difficult phases and he's fantastic to have around.
"I have to say the chances we created against Fenerbahce and West Ham, also I remember at Crystal Palace, we created so many bigger, better chances than today. We had the luck with the post and the deflection. Sometimes, it's what you need. It makes football a cruel game sometimes.
"As I said, we created much better and bigger chances in past games than today so, hopefully, the luck has turned today and we can build on this victory."
The Dutchman added: "We scored some great goals. The first one was an absolute beauty from Casemiro. I just enjoyed the moment, as it was in the top corner, and an amazing goal. He's an amazing person and player and it's fantastic to have him around. Garnacho's was a very good finish, from Diogo Dalot's cross. I think the crowd goes home after a nice night of football."