Jose Mourinho called on the football authorities to start making video calls "perfect" after his Manchester United was denied a goal while beating Huddersfield 2-0 in the FA Cup on Saturday.
Juan Mata's effort late in the first half was disallowed by the video assistant referee for his knee being offside.
But it took the VAR, who is always based in an offsite studio for English matches, several minutes to rule the goal out.
"It's an experimental period," Mourinho said. "They have to get rid of the bad and make it perfect.
"It should be the referee's decision. WhyIJ Because the referee wants to perform well. I don't think they're happy to make mistakes and if they have the VAR to help them make the right decision, I think the referees are probably happy with that.
"But if the VAR changes their good decisions for bad decisions, I think they're not happy with that. It was a bit frustrating because the moment I saw (referee) Kevin Friend touch his earpiece, I knew something was going to happen.
"That doubt is not the best feeling. If the decision is right, I am super happy with that because I want the truth."