The Argentine star "humiliated" his former coach in front of the whole Blaugrana squad, according to ex-New York Red Bulls coach Hans Backe
The 61-year-old has never previously had any direct association with Barca, but his tenure in charge of the Red Bulls coincided with the stays of Thierry Henry and Rafael Marquez, both of whom were Blaugrana players when the alleged incident occurred.
Backe told Sweden's TV4 that Messi intentionally disobeyed instructions from Guardiola in front of the entire Barcelona squad before a game in 2009, though, he refused to reveal his source.
"It was three hours before the game when all the players were sitting and eating, and Messi said he wanted a can of coke," he said.
"Guardiola replied that no player should drink cola three hours before a game.
"So Messi got up and left, returned a few minutes later with a can of cola and drank it right in front of Guardiola and the rest of the team.
"Imagine what would have happened when a player with the profile of Messi goes against the coach in this way?
"It was a war Guardiola could not win; it was impossible."