The talented Premier League star was having a training session with Argentina, where many of his teammates were simply referring to him as "ginger" due to his red-hair.
Mac Allister, who is now 23, has two senior caps for Argentina, and was defended by seven-time Ballon d'Or winner and captain Messi.
"I trained with him when I was in the Argentinian squad twice, but I didn't play with him," Mac Allister has told The Athletic.
"It was fantastic, the same as in the matches he trains. It's something you cannot believe.
"I remember everyone called me 'Colo', which is ginger in Argentina. I don't like it much and he told the team-mates that.
"He said, 'He doesn't like to be called Colo, so don't call him that!'."
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