The Holland fullback is inside the final year of his current deal and been warned by De Boer not to expect to maintain his first-choice status.
"I spoke with him during the Euros," said De Boer. "He will continue in the team, but chances are that he'll often sit on the bench. I must look to the future.
"He will play, but perhaps not as often. The policy of the club with players who only have a year on their contract and do not wish to renew is clear and I supported by me."