Talksport have reported that the Dutchman has responded slowly to treatment on the injury, picked up during Spurs' 3-1 win over Wigan last month.
Van der Vaart, who has scored nine goals this season, is definitely out of this weekend's FA Cup fifth-round tie with Stevenage and faces a race against time to prove his fitness to manager Harry Redknapp ahead of next week's North London showdown with Arsenal.
Spurs host the Red Devils a week later at White Hart Lane, and if the 29-year-old fails to make the team for the trip to the Emirates, it is widely believed that the best he could hope for against United would be a spot on the bench.
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