Dufner wrote in the Daily Mail: "He is the best player we ever had and he would spend hours on his own after training perfecting his technique.
"I was responsible for him coming here but the coaches and everyone around the club helped make him what he is today.
"I remember going to watch him play for Metz in the French second division and there wasn't one top-flight French club there.
"We went along for four or five games and kept asking ourselves the question: 'Why is nobody else in for him?' He was fantastic. We bought him for £1.2million. That was big money for Freiburg and when he started in Germany I was being criticised.
"People were saying I'd bought the biggest flunky in history. But I went out and said 'You watch! This guy will score more goals than anyone else in Germany and Europe.'
"Of course, I was really not that sure but I'm glad I was proved right in the end. He has outstanding ball control. He can take a ball with his feet that is half a metre above his head."