Ajax boss De Boer admits getting tough with Newcastle, Arsenal target Vertonghen
"I think Jan over the last few months has been playing incredibly well," said De Boer. "He has now seen the light, something we have always hammered home. Not only within but also outside the field.
"His development is very beautiful to see. I have been very critical of him and maybe he was angry with me. But ultimately I think he understands me."