De Boer, one of his country's greatest defenders, has been impressed by the Feyenoord centre-half's campaign in Brazil.
"He's made a big step," De Boer told NOS. "He takes risks in his actions at the back. I used to always do this also, I do like that.
"He always tries to tempt his opponent to attack him."
"It depends on what club he goes to. He must be 100 per cent sure to play. He knows he gets this at Feyenoord. But when a beautiful club comes, you should think seriously about it."
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