Republic of Ireland international Graham Kavanagh faces 10 weeks on the sidelines following knee surgery - his second operation in just four months.
The Dubliner tweaked his knee in the warm-up prior to last weekend's game with Preston and, having impressed since moving from Wigan, now faces another frustrating spell out of the game.
He will play no part in the Republic of Ireland's European qualifier away to San Marino early next month, again frustrating for the player who missed the home fixture against the same opponents in November following a similar injury.
"Kav has got to learn from it," said Sunderland manager Roy Keane. "He probably pushed himself too hard from the previous injury. He's got to learn to give his body time to recover, he's paying for it now.
"I had a feeling he would need the operation - he had one or two tweaks recently. He was in the squad against Preston and he felt it in the warm-up. We just thought 'right, let's go and get it sorted'."
Kavanagh (33) sat out five weeks of the season when undergoing knee cartilage surgery in September.
Despite making a successful recovery, the midfielder has been hampered again of late and Sunderland's medical team deemed it best Kavanagh be re-examined.
Sunderland are hopeful that Kavanagh's former Republic of Ireland colleague Kenny Cunningham will return to training next week following a knee injury of his own.
Stephen Ireland is recovering well from a hamstring strain and should be fit for Manchester City's FA Cup third round match against Sheffield Wednesday on Sunday.
Ireland sustained the injury against West Ham last Saturday and subsequently played no part in the game against Everton on New Year's Day but should now be fit for the weekend.
"Stevie tweaked his hamstring at West Ham, and it's a shame because I thought he has been playing particularly well of late," said Manchester City manager Stuart Pearce of the Irish international.
"He just wasn't right ahead of Everton and we just couldn't risk him, but fingers crossed he will be available for Sunday.
"I hope he is, for his sake, because when you eventually get a bit of a run in a team it's nice to keep it going."
