The Argentina international is known for his attacking ability, but former Barcelona coach Guardiola believes that the 25-year-old would be equally impressive if he played in defence.
"Messi wins the ball playing as the false No.9. If you put him at left-back, he would be just as good. He is the best defender in the world," Guardiola told reporters.
"Before arriving at Barcelona I had never seen Messi play. But Tito Vilanova had great things to say about him."
Guardiola will join Bayern Munich this summer.