The 30-year-old withdrew from action in the 77th minute of Saturday's 0-0 draw with Everton, and will now require surgery.
| FOSTER INJURY WORRIES CLARKE
|10/1||Newport at 10/1 with Paddy Power to beat West Brom in the Capital One Cup
"There's no hiding the fact this is disappointing news as Ben is a key player for us," Clarke told the club's official website.
"He was excellent on Saturday before his injury forced him off.
"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."
The injury has also ruled Foster out of the reckoning for Roy Hodgson's England squad for the World Cup qualifies against Moldova and Ukraine in September.