The 20-year-old shot-stopper was hoping to push for a starting place in Tony Pulis's side this coming season, but will be forced to sit on the sidelines after undergoing surgery on the problem.
Foster says that he was playing tennis with his brother when he suffered the injury.
"It was a freak thing - I was playing tennis with my brother and ran back from the net to return a lob, jumped up to hit the ball and as I landed I felt my knee pop out and then pop in again," Foster told the club's website.
"I knew I had done something bad, I just fell to the floor and lay on my back.
"I'm gutted because I was hoping for a good pre-season and to push for the first-team and that's all been blown out now.
"I need reconstructive surgery on my anterior cruciate ligament, the same as Krissy Commons, and the doctors say it'll be about four months to high-intensity training and six months to playing again.
"I've spoken to Krissy and he's told me not to worry. He says he's fine now and his operated knee is stronger than his other one, which is encouraging to hear."
Meanwhile, Stoke will hand a trial Dutch winger Leo Koswal as the Excelsior wide man is available for nothing.
The 27-year-old right winger is looking to join up with fellow Dutchman Peter Hoekstra at The Britannia Stadium.
