Messi says Guardiola transformed the club from the moment of his appointment.
"Guardiola changed everything," he told ESPN Deportes.
"We were coming off of two years without winning anything, the mood in the dressing room wasn't good; it was sad. He is very intelligent. Not only did he prepare us for matches brilliantly, but he also knew what to say to us on an individual level.
"You went into every match knowing what you had to do, you knew what had to be accomplished to get the job done. Guardiola knew how each and every opponent was going to play. He always prepared us to build the play up from the back and focus on going forward. That's a beautiful thing for players that like attack-minded football."