Abdoulaye Doucoure is facing months on the sidelines with a foot injury, with the Everton midfielder set to have an operation soon.
The Frenchman has been one of the Toffees' best players this season, forming a formidable partnership with Allan while contributing two goals and four assists in his eight Premier League games so far.
However, he has suffered a broken metatarsal and requires surgery, and his absence is another issue for manager Rafa Benitez to resolve.
The Spaniard is already without Andre Gomes and Fabian Delph, while Dominic Calvert-Lewin has not featured for two months due to toe and thigh injuries. Richarlison has also been struggling, but he may recover from a knee problem in time to face Watford on Saturday.
Andros Townsend and Demarai Gray have stepped up in the absence of Everton's top two attacking talents, scoring three goals apiece in the league so far this season.
Doucoure, who missed seven games last season with a foot fracture, may be replaced in the starting XI by either Tom Davies or Jean-Philippe Gbamin.
While Everton are yet to officially release a statement regarding the injury, if Doucoure were to miss a few months of action, it would leave him racing to be fit in time for the club's Christmas schedule. They face Chelsea, Leicester, Burnley, Newcastle and Brighton in the space of 18 days.
Benitez's side are currently eighth in the Premier League, having won four of their eight games so far. More recently, they picked up a point at Manchester United before losing at home against West Ham on Sunday.
Source : 90min