The winger needs to see a specialist and boss Andy Scott told the clubs website about the bitter blow.
"It isn't very good news at all, Gareth is going to be out for all while," said the gaffer.
"We got the scan back and the results aren't great but we are still waiting to see the consultant, we also need to see the specialist next Tuesday, so the full extent of the injury will be discussed then. At the moment we are looking at the New Year which is a big blow because Gareth has been fantastic for us."
The cluib are still to officially announce the loan signing of Brentford winger Sam Wood, who has been lined up to cover for Evans' injury.