Scans have revealed that the 33-year-old has ruptured the anterior cruciate ligament in his left knee and is likely to be sidelined for nine months.
Baggies boss Steve Clarke told the club's official website: "I'm devastated for Zoltan. He's a terrific guy and professional and I want to thank him for his contribution this season. It has been invaluable.
"He'll now get our full support during the rehab process. Zoltan has already proven he can come back from this injury once and produce