"Wilf is a very robust player, the first couple of days (after the injury) are very important to have a rest and, from now on, I think he's going to start rehab," De Boer said.
"We don't know how long it will take.
"The risk is that you start too early, that he can have a setback, so we are going to see and don't put a time on it right now."
De Boer admits he wants to bring in reinforcements before September 1 transfer deadline, although he would not be drawn on specific names.
"We're looking very hard and if something happens you're (journalists) the first to hear it," he said.
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