Hammers physio John Green told the club's official website: "He was in Australia on the Olympic trials with the U23s and on Sunday he twisted his cartilage as he was crossing the ball.
"He had an arthroscopy which also revealed what hadn't shown up in the scan, that he had ruptured his cruciate - which is a different ball game altogether.
"It is a bad scenario. He will have reconstruction on his ligament, the same one that he damaged earlier in his career, and will be out of action for about nine months."