The skipper sustained cruciate knee ligament damage during Saturday's game against Burnley.
http://www.nottinghamforest.co.uk/news/article/cohen-out-for-season-1194404.aspx?
***Update*** Cohen has since told the club's official website: "I wanted to take this opportunity to express my gratitude to every fan who has offered me their support since the news broke of the severity of my injury. The amount of messages I've been sent is extremely humbling."