The Republic of Ireland international has not kicked a ball in anger since Euro 2012. It has left Villa shorn of experience in the back four, with the sale of James Collins to West Ham and the loaning out of Stephen Warnock and Alan Hutton to Bolton and West Ham respectively.
Lambert said: "Richard went for a test on Saturday and then another one yesterday. We are just trying to find out what we think it could be. It is really frustrating. We are unsure at the moment what is this ongoing problem. It has come from the groin area/hip area as such."
He added: "That is what we have to try and nail down and put to bed one way or another to try and get to the bottom of that.
"He is still a long way off (from being available) and it is frustrating for Richard.
"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 to the new year, the way it is going at the moment.
"It is frustrating with the other defensive (injury) problems because the squad isn't the biggest in that area."
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- Stephen Warnock