Monga came on in the 74th minute to become the second youngest debutant in Premier League history. At 15 years and 271 days old, only Ethan Nwaneri made his debut with Arsenal at a younger age.
Monga took the pitch in a shirt not showing Leicester's kit sponsor, BC.GAME, due to the company being a crypto gaming platform.
Van Nistelrooy said afterwards: "You could see glimpses of his great qualities. He's a great winger and has speed. He's a fantastic talent, a great boy. He deserved these minutes and hopefully, more to come.
"It's clear and we've been saying that. We want to show what we're about and show our qualities but we aren't able to.
"I want the club to do well. The club is first and that is something to consider. It is hard because I came to the club to help it forward but it is clear there haven't been enough results to get points on the board and that's not good enough."