With VAR not in use for the competition, Leicester were left fuming with the manner of Harry Maguire's injury-time winner for United.
Speaking after the 2-1 defeat, Van Nistelrooy fumed: “It’s a hard one to take because the game was decided on a mistake. It was clear. It was not a matter for VAR where you have to look at millimetres. It was half a metre. It was clear.
“The team didn’t deserve to lose in this way. They fought, they played well, they dominated the game for a long bit, leading 1-0. That’s how you feel after this.
“You can’t turn it back. We have to live with it. We’re all humans and make mistakes, but at this level it’s not good enough.
“When you prepare the whole week for this game and are reacting after the Everton loss and put in a performance like that and it’s been decided on a clear and obvious mistake, that is unacceptable and unthinkable at this level. But it happens to you. We have nothing else to do but pick ourselves up from this. That is not what the team deserved.”
However, the Dutchman also said: “For me the general performance was a big positive.
“After Spurs there was hope, the hope was back. After Everton, the hope was gone. We wanted a reaction after that.
“We said we weren’t good enough against Everton. We showed today that that was an incident between two very good performances against Spurs and United. Also, the belief that we are capable to compete against the best teams away from home as well, that’s the way forward. When we are consistent in that, and that’s our goal, then we can compete at this level.”