Fabregas has ridiculed criticism of Messi's recent performances.
"I haven't heard any arguments," he said. "He's my age. If you see the progress he's made, I don't think I'll ever see it again. He's won everything, except the World Cup.
"He scores goals every game, makes them, always plays. He's a fabulous player, physically and mentally.
"If there are doubts about Messi, I don't know how they see the rest of us. I've never played with a player like him, either at youth level or professionally.
"He's humble, never hides, always gives everything for the team, and loves the club. There's nothing more to say."