The 26-year-old defender picked up the injury during Saturday's draw with Burnley and doctors have told him that he will miss the rest of the 2013-14 campaign.
The injury however is not a reoccurrence of the knee problem he suffered in 2011 which kept him out for 11 months.
The news comes as a cruel blow to Forest manager Billy Davies who has already lost Danny Collins, Kelvin Wilson and Greg Halford to injury.
Davies has already been looking to bring players in on loan to ease the injury crisis, but so far the Forest boss has had no luck in getting recruits.
"I have tried (to bring in players on loan) over the last couple of days. We have made several enquiries. Unfortunately, at the moment, nothing is happening," said the Scot.
"But we will keep looking at things. We will do the best that we can.
"It is unusual having these injuries (in one area of the pitch), but that is football. There is nothing we can do about it."
Cohen has also spoken about his injury and praised the club's supporters for their messages of support.
"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," he told Forest's official website.
"The amount of messages I've been sent is extremely humbling.
"Playing football is my favourite thing in the world. To not be able to do it for so long is going to be so tough, and to be honest it probably hasn't sunk in yet.
"But when you get the kind of support I have from fans, friends and family it makes the long road that little bit easier. It's an amazing feeling and it keeps me motivated.
"I expect to be at the Reading game on Friday night and will be supporting the lads all the way."