"Graham took a whack in the game against Nottingham Forest at the weekend and he has aggravated it since then," Lawrence told the Western Mail.
"He's been in some pain and, for that reason, he hasn't been able to take part in training. So we sent him for a scan to try to get to the bottom of the problem.
"Until we get the results of that scan it's difficult to say how long he'll be out for, but he's got to be a doubt for the Derby game."