The attacking midfielder has returned to the AMEX from a loan spell with Boca Juniors.
Asked about getting called up to the Argentina squad, Mac Allister told The Argus: "With the senior squad, there was a big difference between playing in the league and the national team.
"I was so pleased to be around these players and they treated me very well."
When asked how he got on with Messi, he said: "We spoke a bit. I'm quite a shy person and he is too.
"It was the first time I'd seen him and he was quite quiet. He was very calm.
"We crossed paths and swapped a few words but very little."