Gera injured his left knee during Saturday's FA Cup tie at QPR and scans have shown the Hungary international has ruptured his anterior cruciate ligament.
The 33-year-old sustained a similar injury in November 2011 which kept him out for the remainder of last season. Gera is scheduled to undergo surgery to repair his knee next week, with the usual rehab time for this injury being six to nine months.
He said: "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 at the highest level, and I have no doubts he will do so again."
Gera has played a major part in Albion's fine form during the opening half of the season, which has seen them sustain a spot in the top half of the Barclays Premier League table.
He has netted four goals and also returned to international action with Hungary.
Clarke must now decide whether to look to strengthen his decimated midfield department during the January window.
Youssouf Mulumbu is on African Nations Cup duty, Claudio Yacob is sidelined with a hamstring injury and Graham Dorrans wants to leave the Baggies.