Ruben Loftus-Cheek could be sidelined for several months with an ankle injury.
The 21-year-old Chelsea midfielder, who is on a season-long loan at Crystal Palace, has not played for the Eagles since 28 December.
His injury was initially thought to be a fairly minor one, but it is now feared he could require surgery.
It means the England international is a doubt for this summer’s World Cup.
“The injury is more severe than we originally thought, or perhaps hoped, it would be,” said Palace boss Roy Hodgson.
“I would be very disappointed, surgery or no surgery, if he’s not playing before the end of the season.
“We certainly want him to be and I would presume Chelsea would want him to be because they might like to have a player who’s going to be selected for England in their squad, but there’s literally nothing we can do about it.”
Chelsea youngster Izzy Brown was recently ruled out for the rest of the season after suffering a knee injury while on loan at Brighton.