The 24-year-old was injured during the Eagles' 3-0 defeat to Huddersfield Town on the opening weekend of the Premier League season.
Zaha has already missed two fixtures, but De Boer is in the dark over how long he will be without the former Manchester United player.
"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," Sky Sports News quotes De Boer as saying.
"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."
Palace are already on the back foot, having lost back-to-back matches in the Premier League.