Former FIFA-listed referee Bernd Heynemann feels the VAR needs to be fine-tuned and June's World Cup is too soon.
He told German magazine Kicker, it will be "simply impossible with only six to eight weeks of preparation" for refs to use the VAR system successfully during high-pressure World Cup matches.
Whether the VAR should be used in Russia this June will be debated at a key meeting of the International Football Association Board (IFAB) in Zurich on Monday.
A final decision is expected to be made by FIFA in March.
The aim of the VAR system is to eradicate blatant refereeing errors -- for example, Diego Maradona's infamous 'Hand of God' goal at the 1986 World Cup finals.
The system is being tested in Germany's Bundesliga and Italy's Serie A this season.
Fans complain about long delays leading to confusing -- and in same cases, plain wrong -- decisions made by the VAR during league games.
The system has just started being used in some cup games in England and the Spanish league will follow suit for 2018/19.
- PR disaster -
The system suffered a public relations disaster in Germany last November when Hellmut Krug, the VAR project manager for the German FA, was removed as supervisor.
His demotion came immediately after German daily Bild alleged he had influenced the VAR to intervene to the benefit of Schalke, his home town club.
The technology also regularly broke down in the opening weeks, which dented further confidence in the system with one eye on the World Cup.
"We are going to send referees to Russia who have never worked with the VAR before," former Swiss referee Urs Meier, who officiated at the 1998 and 2002 World Cup finals, told Kicker.
"It is impossible to prepare with just a few trial exercises, because they in no way account for the speed and real-time pressure of a World Cup match.
"The VAR is like an airbag: it can help in an emergency, but only in case of a real emergency."
Mistakes have been made even though referees in Italy and Germany trained with the system for a year before using the VAR during league matches.
Just like in Germany, there have be complaints and controversial scenes in Serie A, but the idea of using the VAR at Russia 2018 has won support in Italy.
- Italian support -
A national training unit is about to open in Coverciano, Florence, where FIFA will send referees will learn how to use the VAR system.
"FIFA has selected Coverciano as the pilot centre and World Cup referees will come here to get ready to use this new technology," said Carlo Tavecchio, the outgoing president of the Italian FA, proudly.
The feeling in Italy is that the VAR has passed the point of no return.
"Those who have not yet taken to VAR will do eventually - we can not go back," said Marcello Nicchi, president of Italy's union of referees.
"Out of 900 incidents (in Serie A), five mistakes have been made."
Yet many top players are unimpressed.
Germany internationals Sami Khedira and Sandro Wagner gave the VAR system a clear 'thumbs down' last November.
"The implementation has been a disaster," said Bayern Munich striker Wagner.
"We have to wait two minutes to get a decision and despite the video, too many mistakes are still being made."
- 'takes too long' -
"It's no longer football... it's turning into water polo."
"It takes too long."
FIFA president Gianni Infantino has said he wants VAR technology at 2018 World Cup
"We can't imagine a World Cup in 2018 being decided by a referee's error," Infantino said last October.
Many feel FIFA will not back down.
"I can't imagine that the people who have implemented this project will suddenly say 'no'," predicts Germany's Markus Merk, who refereed the finals of the 2003 Champions League and Euro 2004.
"That's why I'm convinced the VAR will be used at the World Cup in Russia, but I don't think it will give a satisfactory result."