The 31-year-old England international underwent reconstruction surgery on his anterior cruciate ligaments on Friday after the damage was discovered during an exploratory operation.
Foster, who has won eight caps for England and was a member of Roy Hodgson's World Cup squad in Brazil last summer, will be out until the autumn.
"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," Albion's director of performance Dr Mark Gillet told the official club website.
"But he has had a cruciate reconstruction of his left knee following the injury against Stoke and he will be out for six months."
Foster limped out of Albion's 1-0 win at Stoke last weekend and an initial examination suggested cartilage damage and a month's absence from duty.
But it was agreed that if surgeons found more substantial damage during surgery repairs would be carried out at the same time and Foster, who suffered similar damage in his right knee early in his career, awoke to be told of the lengthier absence.