Sunderland manager Sam Allardyce has confirmed defender Younes Kaboul will be "out for a long time" through injury.
The centre-back tore his hamstring in the side's 3-1 loss to Chelsea last Saturday.
Despite suffering the injury during the game, Kaboul managed to play out the 90 minutes at Stamford Bridge.
But in a blow to Sunderland's bid for Premier League survival, Kaboul has been ruled out of the busy festive period.
"Younes is out for a long time," Allardyce said. "It's going to be weeks, maybe more, and it's a big blow to us.
"The last place we want to be weakened is down our spine, the core of the team.
"It actually was quite surprising he carried on with the extent of the injury. Maybe it was the adrenalin running through him at the time, but carrying on he may well have made it a little worse."
Sunderland, who sit second-last in the table after 17 games, travel to Etihad Stadium to face Manchester City on Boxing Day.
Allardyce will also be without Jack Rodwell for that game due to suspension, with Lee Cattermole touch and go after sustaining a back injury.
"I'm shocked Jack's suspended," Allardyce said. "He must have been booked almost every time he played. I've spoken to him about his tackling and decision-making."