The Dutchman is unlikely to play again this season, but De Boer has no doubts about his quality.
He told talkSPORT: “He has made a transition in a positive way.
"When he was at Groningen he did OK, he was physical, but we Dutchmen always want defenders to play with the ball and that is something we thought was not his best thing. But now he does that.
“His passing is excellent but, most importantly, his defending is outstanding. He is showing it week in, week out in one of the toughest leagues in the world."