Watford head coach Javi Gracia accepts Abdoulaye Doucoure could follow former team-mate Richarlison out of Vicarage Road.
Richarlison joined Everton ahead of the 2018-19 season to reunite with former Hornets boss Marco Silva, and Doucoure has been linked with signing for the Toffees in the January transfer window.
Doucoure, who signed a new contract running until 2023 in August, is also reportedly a target for Ligue 1 leaders Paris Saint-Germain and Arsenal.
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Club owner Gino Pozzo has indicated Watford will fight to keep the midfielder, who has started all 19 Premier League games for the club this term.