Sunderland midfielder Jan Kirchhoff will be out for three months after undergoing knee surgery.
Kirchhoff suffered a cartilage tear in the recent 1-0 defeat to Chelsea.
The German midfielder saw a specialist last week and has now undergone surgery, with David Moyes confirming he will be missing for 12-weeks.
Midfielders Lee Cattermole, Paddy McNair, Duncan Watmore and Lynden Gooch are also long-term injuries for Sunderland.
Meanwhile, Billy Jones (hip), Javier Manquillo (knock) and Steven Pienaar (calf) have returned to training at the Academy of Light.
But they are unlikely to make the squad that travels to Manchester United on Boxing Day.