"Yes, [Van Dijk can become the best defender in the world]," he told Sky Sports. "I think he could play for Barcelona, I think he could play for Real Madrid - it wouldn't surprise me if he took his career on to those pathways.
"The first day he came into Celtic I pulled him to one side after training and said 'you enjoy yourself here because you're not going to be here long'.
"I couldn't believe we got him for €2.2million. He was outstanding. I couldn't believe that none of the big clubs took a bite at him before Southampton.
"I believe he could play at any club side in Europe now - he's that good. He's only 24, he's got 10 years in him. He has got everything you want from a modern-day centre-half: he's powerful, he's quick, technically brilliant, he reads the game well, superb in the air.
"So it doesn't surprise me that the big clubs are coming in now - it's just surprising that they have waited so long."
Van Dijk made 21 league appearances in the 2016-17 season, but injury meant that he missed the back end of the campaign.