Man United boss Jose Mourinho has claimed the decision to rule out Juan Mata's goal in the FA Cup went against protocol

Jose Mourinho was bemused by the video assistant referee's controversial intervention as Manchester United secured safe passage to the FA Cup quarter-finals at Huddersfield.

Romelu Lukaku netted early in both halves at the John Smith's Stadium to secure the visitors a 2-0 win on a night when technological teething problems reared their head once more.

Juan Mata wheeled off in celebration after netting from an acute angle just before half-time, only for referee Kevin Friend to eventually chalk the goal off as the narrowest of offsides was revisited.

United boss Mourinho is happy to see technology utilised and pointed to its relative success in Portugal, but felt it was wrong for video review assistant Neil Swarbrick to get involved given it was not a clear and obvious error.

"It is amazing when the truth comes," the United manager said.

"But also (there are) problems and one of the problems is exactly what happened today, which goes a little bit against the words of the protocol.

"The words of the protocol are something like a clear and obvious situation and for sure it was not a clear and obvious situation.

"But we know that it is experimental, and we have to accept. But I am speaking after a 2-0 victory."

Mourinho laughed so was then asked whether his view would be different had it cost United.

"I am close to winning the best-behaved manager in the Premier League, so nothing that can make me lose that trophy this season," he said, smiling once more.

"The feeling is a bit strange and I was sharing it with David (Wagner, the Huddersfield boss) on the touchline and (fourth official) Anthony Taylor, which is the moment of the doubt.

"We score a goal, I cannot be happy.

"David concedes a goal but he doesn't know if he goes to half-time with 2-0 or 1-0, and it takes time and then the decision comes.

"So, we have to adapt to that waiting time and cope with the frustration when that decision goes against you.

"At half-time I had to say 20 times to my analysts and assistants to shut up because I don't want the players to know what happened.

"I want the players to feel, 'Yes, it is offside and to move on', without being sad or unhappy.

"'Shut up, we have to move on', but one day it will go in our favour."

United midfielder Mata said it was a "very tight" decision, but was philosophical about the decision, whereas Huddersfield boss Wagner is not a fan of VAR at all.

"The emotion was killed," the Huddersfield boss said.

"I said this immediately to Jose during the incident that, 'Whatever will now be decided, whether this decision goes in our favour or not, I don't like this', and I've said this before.

"The emotion is killed, the emotion in the stadium in the stands, for us managers in the dugout. This is why I don't like this system. Maybe I am too traditional."

Wagner's side put up an impressive first-half fight before Lukaku ended the tie clinically, with United's reward a quarter-final against Brighton at Old Trafford.

"I am happy that we are in the quarter-final," Mourinho added.

"I am happy we won a match, especially after the defeat (at Newcastle last weekend).

"It was the kind of game where a draw would not be a drama to take the match to Old Trafford, but the game was difficult.

"I think they had a different approach to what they had in the Premier League."