Abdoulaye Doucoure is set to be sidelined for Everton until early April.
Doucoure suffered an unspecified injury problem which forced him to sit out the Blues’ defeat to Champions League qualification rivals Chelsea on Monday.
The Frenchman has become an integral part of Carlo Ancelotti’s side this season since completing a £20 million switch from Watford last summer.
Ancelotti could not shed any light on the problem which sidelined Doucoure but confirmed he will be unavailable until after March’s international break.
“We don’t really know what happened,” admitted the Everton manager.
“He will be out until the [international] break. It’s a quite important injury.
“I don’t know how long he will be out. We will manage his absence.
“I think we have the possibility to replace him well.”
Reflecting on the end of Everton’s three-game winning streak hand nine-match unbeaten away run, Ancelotti conceded his team were second best to Chelsea.
The 2-0 reversal saw the visitors lose ground in the race for a top four finish as Thomas Tuchel’s side extended their own advantage to four points.
And while Ancelotti felt that the scoreline was a fair reflection of the game, he insisted it does not alter Everton’s ambitions for the remainder of this season.
He said: “They [Chelsea] played better than us. They played with more quality. When the opened the scoring, the game was more difficult.
“This defeat doesn’t affect our dream, our target. We lost but we are only four points behind them with a game in hand.
“We have to be focused, because these next games [against Burnley, Crystal Palace and Brighton] are important.
“We are disappointed, but not too much. We have to try to get a result in the next games.”
He added: “Honestly, we haven’t talked about the top four. Our target is to be in Europe.
“We are still in the fight even with this defeat. We will have other chances to take.”