Messi has struggled to replicate club success with Argentina, despite being close to lifting silverware with the national side on several occasions.
The Barcelona superstar saw La Albiceleste finish runners-up at the 2014 World Cup in Brazil and consecutive Copa Americas in 2015 and 2016 against Chile.
At the 2018 World Cup in Russia, Argentine were knocked out by eventual winners France at the round of 16 stage, and had to settle for third place at the 2019 Copa America.
Ustari believes it would be unfair if Messi fails to hoist the World Cup before his retirement, and reveals the five-time Ballon d’Or winner becomes sad when his country fails to win.
"I have no doubt that if Messi isn't world champion, football will be unfair," the 33-year-old told Radio Mitre.
"Doubting Messi is nonsense. Things can go well or not, but I can tell you that I have seen him crying like a child because of what was happening in the national team. It's very difficult to be in his shoes.
"I remember that in the 2011 Copa America I was recovering from my knee and, when they lost, I went to see Messi at the Ezeiza venue and I saw him like I had never seen him before, devastated."
Ustari played alongside Messi at the 2008 Olympics in Beijing when Argentina won the gold medal after beating Nigeria.
