Foster suffered his latest setback against former club Stoke City earlier this month and had successful surgery ahead of his lengthy rehabilitation.
"I know what to expect," Foster told West Brom's official website.
"I've been here before. You can't push cruciates - you have to do six months. That's a must.
"There is no chance of coming back early so I know it's at least October before I return. You follow the guidelines of what the physio says and take it from there.
"October would be the six-month target so fingers crossed it will be around then."
Foster's previous cruciate injuries occurred in 2003 and 2007 when he was at Stoke and Manchester United respectively.
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