Harris, 31, sat out Saturday's victory over Leicester City after aggravating the problem during training.
Physio Bobby Bacic told the club's official website: "Neural or nerve pain in that area appears to be very much like a hamstring tear which is why we immediately had a scan done.
"However, the scan has revealed that there is no hamstring tear so we are left dealing with a neural problem which is notoriously unpredictable in terms of the time it takes to heal properly.
"At the moment, we just have to take things one day at a time.
"We can't afford to take risks with this sort of injury so Neil could be out of action for four to six weeks even."