Zoltan Gera will be out for the rest of the season and will miss between six and nine months of action.
The news is a big blow to West Bromwich Albion, who will have to look to alternative options in their squad after the 33-year-old was found to have ruptured the anterior cruciate ligament in his left knee in his side's 1-1 FA Cup clash with Queens Park Rangers on Saturday.
Baggies boss Steve Clarke confirmed the news, saying: "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."
Gera has been one of the key figures in West Brom's excellent performance this season, and at 33, will have question marks over his West Brom career, but Clarke was confident he would play for the club again: "Zoltan has already proven he can come back from this injury once and produce at the highest level and I have no doubts he will do so again."