Unai Emery believes he missed out on the chance to become the world's best coach during his time at Paris Saint-Germain because there was no VAR.
The Spaniard won seven domestic trophies in two seasons in the French capital but left in 2018 after failing to mount a serious Champions League challenge.
In their first season under Emery, PSG thrashed Barcelona 4-0 in the round-of-16 first leg only to suffer an astonishing 6-1 defeat in the return match at Camp Nou.
A year later, they reached the same stage of the competition only to lose 5-2 on aggregate to Real Madrid, who went on to lift the trophy for the third season in a row.
Emery thinks his fortunes as an elite-level coach might have been better had VAR been in existence for those ties.
"In Paris, I missed the chance to become the best coach in the world," he told France Football.
"I won the league, four domestic cups and two Trophees des Champions. But the ultimate goal was the Champions League.
"The first year, in the round-of-16 against Barcelona, were played a really high-level first leg. In the return leg, we were knocked out because VAR didn't yet exist. We were very clearly knocked out by refereeing decisions.
"The second year, against Real Madrid, we bowed to a team who recorded an historic hat-trick and, again, in the first leg, we could find fault with the officiating.
"To conclude: we lost the first time on refereeing decisions, and the second against the defending and future champions."
The 4-0 win over Barca came despite Thiago Silva being left out of the starting line-up.
It was initially reported the Brazil centre-back had an injury but reports soon suggested his omission was more of a tactical decision.
Emery has now explained he had grown frustrated with the player for not following his instructions to hold a higher defensive line.
"I wanted the team to defend higher. Thiago Silva is a great player, but I wanted him to be higher and I couldn't get him to accept that," Emery said.
"I wanted him to get out of his comfort zone, to dare to defend higher so that the general pressure of the team on the opposition would be more effective. I worked with him to make him accept this, but I didn't succeed.
"This characteristic of Thiago Silva's game reflected on the whole team, which, under pressure, had a natural tendency to drop deeper."
Despite a disappointing end to his two seasons in charge, Emery insists he always felt he had the backing of president Nasser Al-Khelaifi.
Indeed, the former Arsenal boss says it was left to him to convince Neymar of PSG's plans ahead of his world-record €222million transfer from Barcelona
"I always felt supported by president Nasser," he said. "For example, when we were about to sign Neymar, it was up to me to explain in person to Ney that we were going to build the team around him.
"With such a phenomenal player, you can't tell him there is already a team there and that he will have to adapt. You have to create the team for him. Otherwise, he wouldn't come - that's for sure.
"The president and the directors discuss the contract, but it's the coach who has to convince him of the game plan."