Watford captain Troy Deeney will be out for several weeks after undergoing minor surgery on his knee.
The striker, who has played every minute of his team's opening Premier League games, could miss up to six matches.
The Hornets have lost both games and are yet to score a goal, sitting bottom of the table.
"We will miss Troy because he is a very important player for us," said Watford boss Javi Gracia.
Deeney scored 11 times last season as Watford finished 11th and reached the FA Cup final, with his last goal being an injury-time penalty in the FA Cup semi-final win over Wolves at Wembley.
Meanwhile, the club's Swedish international winger Ken Sema, 25, has completed a season-long loan move to Udinese.
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