The former Celtic star has hailed the importance of the influential midfielder, and feels the Catalans are vulnerable in defence at the moment due to injuries
The former Sweden international played alongside both players during his two-year spell at Camp Nou between 2004 and 2006, and still keeps an eye on the Blaugrana's progress.
"[Messi] was only a kid back then, but already an excellent player. Anyone who says that he knew Messi would become this good is lying, though," he told El Pais.
"He has skill, vision and is always in control. He is a phenomenon and deserves every award that he wins.
"However, Xavi is the soul of the club in my opinion. He never misses a pass. He plays in a very difficult position and doesn't score many goals. He is the key to Barcelona's success, though."
The 41-year-old, who scored over 170 league goals for Celtic, then went on to discuss the upcoming Champions League clash between the Hoops and Barca, and feels the Blaugrana are vulnerable at the moment due to their injury troubles at the back.
"Right now you can upset Barcelona a little because they are without Carles Puyol and maybe Gerard Pique too.
"Daniel Alves has only just come back so it is not the back four that Barca would prefer to have.
"You saw it in the first game at set-pieces they were a little bit short and there is no doubt that they are missing Pique and Puyol who are used to competing in the air."
Celtic were beaten 2-1 by Barca at Camp Nou two weeks ago thanks to a last-gasp Jordi Alba goal.