After Erik ten Hag was dismissed as manager on Monday, his fellow Dutchman stepped into the interim role before a permanent replacement is appointed.
That permanent manager is set to be Ruben Amorim, but the Sporting boss is yet to complete his move to Old Trafford.
Van Nistelrooy may not be in charge of the Red Devils for long at all, but his stock is already high after the demolition of the Foxes in the last 16 of the Carabao Cup.
Casemiro opened the scoring with a stunning long-range strike, with the Brazilian scoring twice, Bruno Fernandes also netting a brace and Alejandro also getting on the score sheet.
The United boss was thrilled to see the goals flying in, but he felt that his side benefited from plenty of luck along the way, perhaps balancing out from a lack of fortune they have experienced in recent games.
‘I have to say that we scored some great goals, the first one was an absolute beauty from Casemiro,’ Van Nistelrooy told Sky Sports. ‘I have to also say we had some luck as well. Two posts and it comes to Casemiro’s feet. Bruno with the free-kick, it’s deflected and it’s in.
‘Five goals, but as I said before, the chances against West Ham, Fenerbahce, Brentford, we created so much more, bigger chances and we couldn’t finish, but all of a sudden we had some luck on our side and that made it a great day today.’
Leicester scored two goals of their own but never really worried the home side as they blasted back into a comfortable lead soon after conceding.
‘I think it’s always the target for us to react and go straight back to attack ourselves,’ said Van Nistelrooy. ‘The reaction of the players after these moments were excellent and they deserved the credit for a good win and happy that the crowd goes home with a nice night of football.’
The temporary boss picked out Casemiro as a standout player, saying of the Brazilian midfielder: ‘I was delighted for Casemiro, the way he’s been training and helping the team, the club, it’s such an example.
‘I’m delighted to have him around and going through difficult moments… He got a lot of criticism after the Liverpool game.’