Having had time to analyse replays of the goal, VAR would most likely have overturned the on-pitch decision.
However, the Football Association announced in mid-December, external that the technology would not be used in the FA Cup until the fifth round.
“VAR has only been used in the FA Cup previously for matches at Wembley Stadium and at Premier League grounds because of the infrastructure, workforce and costs that are required for its operation,” the FA said in a statement on 16 December.
“This decision ensures that there is a consistent refereeing approach for all clubs taking part in the same stage of the competition.”
Manchester United manager Ruben Amorim admitted his side were fortunate with the decision, and feels it vindicates the use of VAR, even if his side benefited on this occassion from its absence.
“With VAR it’s not a goal,” he said.
“I think it’s important to have VAR because it’s fair for the game. It’s hard to lose like that with an offside play. But we deserve a little bit of luck.”
Former Manchester United midfielder Roy Keane felt the debate is not over whether VAR is needed, but that it was a clear enough error that should have been picked up by the officials.
He said on ITV: “That’s a poor decision, the linesman [assistant referee] has got to see that.
“You have got to get those big decisions right.”
Former Arsenal striker Ian Wright added: “Look where the linesman is. That impacts him [Van Nistelrooy] that impacts his managerial career because it’s another loss.
“That would have been a really big confidence booster for them. But they have to lift themselves, because of the officials.”
Van Nistelrooy added: “It is a hard one to take because the game was decided on a mistake.
“It is not a matter of VAR where you look at millimetres, it was half a metre, it was clear.”