Unzue, who was assistant to Luis Enrique between 2014 and 2017 at the Camp Nou, said the standards demanded by the Argentine are too much for some teammates.
– Stream LIVE games and replays on ESPN+ (U.S. only)
– Insider Notebook: Ozil eyes MLS; Spain‘s ‘shameful’ sandwich
“What has kept Messi [where he is] is ambition and how to live with moments of stress,” he told YouTube channel Idolos.
“He self-demands to himself to keep being the best and, as a result, also wants the best around him, teammates, physio, or coach. That produces a great stress that some teammates are not capable of bearing.”
Messi almost left Barca in the last transfer window amid speculation of a move to Manchester City and many expect him to leave the club when his contract expires in June.
The 53-year-old Unzue said the situation surprised him.
“The truth is it came as a shock to me that Messi wanted to leave but, knowing the club and Leo, I understand that he might have very, very important reasons for taking this decision.
“What I was convinced of was that if, in the end, he stayed, he’d give his best and maintain his ambitions.
“[Barcelona coach Ronald] Koeman is managing Messi and the group well, and the situation is not easy, although I think entering a big club that has won titles is harder when they’ve stopped winning.
“When we arrived with Luis Enrique we did so after a year the team had not won anything.
“That’s happening to Koeman now, he’s arriving at a messed-up time but if it’s not a messed-up time, you don’t get the call to become coach of Barcelona.”