Walker, who played over 100 times for The Lions in the late 1980s, collapsed after heading a ball during a training session in Belfast, where he was hoping to pick up another coaching badge.
Early indications suggest the the former player had weak spot in his spine and had been suffering from neck and back pain in recent years. He is due to undergo major spinal surgery tomorrow, after which doctors hope to have a better idea of the 47-year-old's condition.
All at Millwall Mad wish him well and hope for a speedy recovery.