The Dutchman brought Bale to Tottenham shortly before his departure in 2007 and attempted to sign him for Ajax two years later when Bale was out of the side.
He said: "I think he is probably the best player in the Premier League, pound for pound.
"I remember him at Southampton when we played there with our reserves. He was 15 or 16 and he was one of the best players then. Nathan Dyer (now at Swansea) was there and Theo Walcott (Arsenal) was there.
"Gareth played left back at the time and the next year he was scoring goals from free-kicks. Walcott then went to Arsenal, Gareth went to Spurs.
"Walcott probably started better than him, but you could see Gareth's potential. His stats were better than all the other players."