UEFA president Aleksander Ceferin has accused the head coaches of clubs including Manchester United, Manchester City, Real Madrid and Barcelona of showing a "lack of respect" after they missed Monday's meeting on the use of VAR in the Champions League.
VAR will be used from the first legs of the round of 16 ties, which begin next week, and the coaches of the 16 clubs were invited to talks in Germany.
But only Massimiliano Allegri (Juventus), Thomas Tuchel (Paris Saint-Germain), Eusebio Di Francesco (Roma), Domenico Tedesco (Schalke) and Bruno Genesio (Lyon) were in attendance.
Manchester United's Ole Gunnar Solskjaer, City's Pep Guardiola, Tottenham's Mauricio Pochettino, Real Madrid's Santiago Solari and Barcelona's Ernesto Valverde were among those who were not there, with several clubs opting to send assistants or other members of staff.
Liverpool could not be represented because they were playing at West Ham in the Premier League on Monday night.
"We called a meeting regarding VAR with the 16 coaches that have qualified for the round of 16, and only five came," Ceferin told reporters in Rome on Thursday.
"For me it's not just a lack of respect for [UEFA's chief refereeing officer] Roberto Rosetti's team and UEFA's organisation, but the influence of coaches is huge and, if they complain about being refereed, they should at least come and see what the experts say about the use of VAR."
Rosetti said it was "a pity" that not everyone had attended, adding: "It was very important for us to speak directly with the clubs, to explain to them how we are planning to use VAR in the Champions League, and how we have prepared the referees for this.
"It is essential that everyone understands when VAR can intervene, but also understands when VAR does not intervene. There were good discussions, and the feedback we have received was very positive."