Everton caretaker manager David Unsworth revealed centre-back Michael Keane has been hospitalised with a leg infection.
England international Keane, 24, was a notable absentee for Sunday's 2-0 loss at Leicester City in the Premier League.
Unsworth - speaking after his first league match in charge having stepped in for former boss Ronald Koeman - said Keane is recovering from an infection in a gash sustained during Everton's 3-0 EFL Cup win over Sunderland on September 20.
"Michael got a knock; a really bad gash against Sunderland, and it got infected and spread up his leg," Unsworth said.
"It's been really, really nasty and he's been hospitalised.
"We got some good news when we got to the ground that there's been a big improvement, so hopefully he can be available for Sunday's game [against Watford]."
Keane has made eight Premier League appearances this term, having arrived from Burnley in the close-season.
Everton are in the relegation zone following their loss to Leicester, behind 17th-placed Swansea City on goal difference after 10 games.