PSV full-back Willems was substituted in the 42nd minute of his side's 2-0 home defeat to Feyenoord with what is described as "damage to the inner tube of his knee."
The 20-year-old has won 11 caps for Netherlands in his international career, his most recent coming in the 2-2 draw with Japan in November last year.
But his prospects of earning a place in Louis van Gaal's squad for the World Cup in Brazil are now in question after interim PSV boss Ernest Faber - in charge while Phillip Cocu recovers from surgery on a tumour in his back - admitted the injury is a bad one.
"Willems has suffered an injury to the inner tube," Faber said in quotes reported by De Telegraaf. "It does not look good."
Netherlands start their World Cup campaign on June 13 against holders Spain before taking on Australia and Chile in Group B.
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