The 24-year-old suffered the injury just two minutes into the opening game of the season, a 1-0 win over Bradford at the City Ground.
He does not require surgery, but will not be able to return until December.
Manager Colin Calderwood said: "There's no operation required and there's no other damage to anything else in or around his knee.
"It's better than we feared because he doesn't require surgery and in the long-term his knee will be sound."