Foster played through pain after coming on as a sub at half time in the win at Hereford, with his injury flaring up further the following day.
Manager Andy Scott said to the Advertiser: “It happened when he was pulled out of the field of play by an opponent and he felt something 'pop' in his knee.
“He carried on but it was swollen the next day and at the end of last week we had him scanned and he saw a specialist and he'd ruptured the cruciate. It's a blow for him and for us because he's an important part of our squad.”
Foster is only on a one year contract after signing from Stevenage in July.