The 33-year-old, who currently plies his trade in Serie B at Monza, played alongside the six-time Ballon d'Or winner in a short-term loan spell last year.
"Training with Messi left me speechless," he told DAZN.
"I had always said that Cristiano Ronaldo was the best in the world, but Messi is something else. He's not normal.
"While training with him, I felt inadequate for the first time in my career. He was doing incredible things. I felt like saying, 'I'm done, I'm going to quit playing!'"