The 33-year-old defender has not featured since the club's summer tour of the United States because of hip and groin problems which show little sign of improvement.
Lambert said: "He is not training at all. He does little bits himself with the physios but I think even that is starting to hurt a bit.
"I would say he is out until the new year, the way it is going at the moment.
"It is frustrating because of the other defensive injury problems we have and the squad isn't the biggest in that area.
"He is still a long way off from being available and it is frustrating for Richard.
"We are just trying to find out what we think it could be. We are unsure at the moment of what this ongoing problem is. It has come from the groin area/hip area and we need to get to the bottom of it."