Diafra Sakho has been ruled out for up to eight weeks through injury and will miss the Africa Cup of Nations, according to West Ham manager Slaven Bilic.
The Senegal forward has featured only twice for the Hammers this season due to a long-term back problem and because of a thigh injury suffered in the 1-1 draw at Manchester United in November.
Sakho, who was originally expected to be out for up to six weeks, was due to represent his country at the AFCON in January.
But ahead of West Ham's trip to Swansea on Boxing Day, Bilic revealed the 26-year-old had undergone back surgery to repair a fresh problem and could be on the sidelines until March.
"We have a few knocks but are expecting everyone to be fit except for Diafra Sakho who will be out for six to eight weeks. Sakho had to do a back operation," he said.
"He will miss the Africa Cup of Nations and it is a big blow for us.
"They're talking about even March for him to be back, so he's definitely ruled out of the Africa Cup of Nations."
Bilic also stressed West Ham would do all they can keep midfielder Dimitri Payet at the club beyond the January transfer window.
West Ham co-chairman David Sullivan said in November he had no intention of selling Payet in the New Year, while captain Mark Noble admitted he would be shocked if the France international left the club.
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The 29-year-old, who signed a new five-and-a-half year deal in February, spoke about the possibility of playing for other clubs, including Arsenal, in an interview with French broadcaster RMC Sport earlier this week.
"We do not want to sell Dimitri Payet and he is a great player," Bilic said. "He has a contract and we will do everything to make sure he stays with us.