"He was being treated, came out training and then got another bang," said Sunderland boss Mick McCarthy.
"We've had it looked at and we've seen on the scans that he's actually got a tear in his cartilage, which is very sad because he'd be a welcome addition to the squad. He would be like having a new player.
"It's dreadful luck, absolutely dreadful. You only have to look at the game he played at Mansfield, when he was excellent.
"It would be nice to have him back because he's got genuine pace and he'll cause people a problem.
"None of the players are happy when they're injured, but he seems to be injured all the time.
"You come in and do your work and go home and you end up not being part of the fabric of it really, and you miss out on that, you miss out on the camaraderie, going to games, having the crack, having a laugh with the lads.
"At the minute, it seems to be an uphill battle for him to get fit."