Physio John Dickens told the club's official website: "He'll be out for around four months.
"The good news for Sean is that once he's had the operation and rehab he shouldn't have any further problems with the knee.
"Sean was obviously a little bit down after being told the news, but he's picked himself back up.
"Sean had worked so hard on his rehab. In fact he's one of the hardest working players I've ever dealt with."