The England international goalkeeper had initially been expected to only miss the next month, with West Brom suggesting the issue was a knee cartilage problem.
However, Foster underwent surgery on Friday and an exploratory operation uncovered a more serious problem, damage to his cruciate ligaments.
Foster had been an ever-present for Tony Pulis' side this season, making 28 appearances in the Premier League, before Boaz Myhill stepped in for Saturday's 3-0 defeat at Manchester City.
The former Manchester United goalkeeper suffered a similar injury to his right knee earlier in his career and will be hoping to once again make a successful recovery as he targets an October return.
West Brom's head of sports science and medical staff, Dr. Mark Gillett, said: "Ben is in good spirits despite this obvious disappointment to him – he's already talking about his rehab and wanting to get started on it immediately.
"But he has had a cruciate reconstruction of his left knee following the injury against Stoke and he will be out for six months."
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