"We've had bad news on Kieran," manager Roy Keane told the club's official website. "He saw a specialist this week and he has a stress fracture of his back.
"We'll have more news after he sees a surgeon later today. I would have thought that he'll be out for two to three months.
"It is a major disappointment. Kieran was able to do elements of training but it affected him when he twisted or hit the ball hard.
"In the last week or two we were hoping that it might settle down, but clearly it's a lot more serious."