West Ham striker Diafra Sakho is facing up to eight weeks on the sidelines with a back injury.
The 26-year-old Senegal international has made only two Premier League appearances this season, and will now miss the crucial festive period and next month's African Nations Cup.
Hammers boss Slaven Bilic, speaking ahead of the Boxing Day trip to Swansea, said: "He had a back operation and he's going to be out for up to six to eight weeks. Of course it will rule him out for the African Cup of Nations."
Sakho last featured during the 1-1 draw at Manchester United on November 27, opening the scoring after 90 seconds before going off midway through the second half.