The 25-year-old is due to undergo surgery after sustaining damage to his tendons and ligaments during last Saturday's 1-1 draw with Oxford United.
"The surgeon said he has seen very few injuries like this, especially in football," manager Phil Brown is quoted as saying by BBC Sport. "It's very disappointing news. Ben has been a key player for us."
Coker has made eight appearances for the League Two side since the start of the season.