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 leader Paris Saint-Germain and Arsenal.
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.
But Gracia, whose side sits 10th in the table, accepts Doucoure's future may not be at Watford, although he hopes the 25-year-old can be persuaded to stay.
"Last season every week we were talking about Liverpool or Everton, all the clubs that were interested in him," he told reporters ahead of Saturday's home game against Newcastle United.
"We can speak about it again. For me Doucoure is a great player, a very important player for us. I do not know what will happen. It doesn't surprise me other clubs want to sign him because he is a very good player.
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"In this moment, he is playing for us and I try to help him improve his level - thinking he is going to be with us for a long time. It is important the club know how important he is - the owner as well. But after that, they decide what is better for the club.
"Sometimes some players, like last season what happened with Richarlison, decide themselves. It could happen again. This is something we don't want, but it is football."