Foster suffered the injury during Albion's 0-0 draw at Everton in the Premier League on Saturday meaning he will take no part in England's World Cup qualifiers against Moldova (September 6) and the visit to Ukraine (September 10).
"There’s no hiding the fact this is disappointing news as Ben is a key player for us. He was excellent on Saturday before his injury forced him off," said West Brom manager Steve Clarke.
“In the meantime, I’ve got two other very good goalkeepers to choose from.
“Boaz did well when he deputised for Ben last season and Luke showed on Saturday he is a more than capable goalkeeper.”