Cardiff manager Neil Warnock said "referees guess a lot" and joined Fulham boss Claudio Ranieri in saying VAR cannot come in soon enough after controversial decisions in their sides' Premier League defeats on Friday.
The Bluebirds were thrashed 5-1 by Watford after Josh Murphy was denied a penalty shortly before half-time.
Fulham lost 3-1 after West Ham forward Javier Hernandez scored with his arm.
"There will be mistakes but they will get more right with VAR," said Warnock.
Video assistant referees have been in use in the Champions League, FA Cup and Carabao Cup this season and will be introduced in the Premier League next season.
"Referees need help," said Warnock. "It is human nature, refereeing is hard. But the quicker VAR comes in… because that would have been solved in 30 seconds.
"Everyone on the pitch knew it was a penalty. It wouldn't take long to have a look."
Hernandez's West Ham goal cancelled out Ryan Babel's opener for Fulham before Issa Diop scored with a towering header five minutes before half-time and Michail Antonio nodded in a third late on for the Hammers.
"It was a pity that the referee didn't see the handball. VAR is very important for next season," said Fulham boss Ranieri.
"It is much better with VAR, I understand that it is not easy to adapt but it is slowly, slowly getting better."
Speaking on BBC Radio 5 live, former West Ham defender Matthew Upson said Hernandez's use of the arm was "intentional" and that the Mexico forward "knew what he was doing".
'He said to me he made a mistake' - Warnock
Cardiff were trailing 1-0 when, in the 43rd minute, Murphy drove into the box and was brought down from behind by Watford's Daryl Janmaat just inside the area.
Watford midfielder Gerard Deulofeu then went on to score a hat-trick and captain Troy Deeney added another two goals as Warnock grew more frustrated with the officials throughout the match.
The Cardiff boss claims referee Simon Hooper told him after the match that he made the wrong decision.
"He said he has seen it back and he made a mistake," said Warnock. "Now I'm 70 years of age and mistakes like that should not be made, not at this level.
"I am disappointed. I like him as a referee, but he's in a great position. His linesman couldn't help him because of their angles, but the penalty is as straightforward as you are going to get.
"Troy Deeney, all Watford's staff, they all told me it was a penalty, which doesn't help really. The referee just said to me he's made a mistake, so what do you say to that? It doesn't help me when we've just been battered 5-1."
Warnock said he had to calm his players down at half-time and he has "never seen" that kind of reaction from them before.
"I had to try and get it out of their minds at half-time. The injustice of it," said Warnock. "I think the referees guess a lot of the time and you can't do that at this level. People know if we go in at 1-1 then it's a different ball game. The players were really down.
"He hasn't made the mistake on purpose. But it doesn't help us, we are kicked in the teeth, but we have to bounce back now.
"Anyone who has played the game knows it is a penalty, I am sure the referee is distraught but that doesn't help me."