Sunderland midfielder Kieran Richardson will spend up to three months on the sidelines with a stress fracture of the back.
Richardson, who moved to the Stadium of Light from Manchester United in the summer, missed the Premier League clash at Old Trafford 10 days ago after complaining of the injury.
Tests have now revealed that the problem is worse than initially thought and he will be forced to sit out for the next few months.
Sunderland manager Roy Keane told the club's website: "We've had bad news on Kieran.
"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. I would have thought that he'll be out for two to three months.
"It's a major disappointment. Kieran was able to do elements of training but it affected him when he twisted or hit the ball hard."