Daryl Janmaat has been ruled out for up to six weeks by Watford due to a shoulder injury.
Janmaat suffered the injury when he fell awkwardly during his side's 3-1 win over Manchester United on Sunday.
The Holland international defender, who joined the Hornets from Newcastle only last month, set up Etienne Capoue's opener in the victory at Vicarage Road before he was replaced after 51 minutes.
"We really didn't need this now as it is quite a bad injury," said Watford boss Walter Mazzarri during Friday's press conference.
"It could be a shoulder fracture - he has been checked - and we will know how long he will be out for in the next few days.
"As you know, we count on him a lot. He was progressing, and started playing in the squad the way I wanted him to play."
Janmaat started in Watford's successive victories against West Ham and Man Utd, but he is now expected to be replaced by Nordin Amrabat for their match at Burnley on Monday Night Football.
Defender Jose Holebas is also a doubt for the fixture, while a number of other players are due to be assessed following Watford's impressive win against United which took them into the top half of the Premier League table.
Frenchman Capoue put the Hornets ahead against Jose Mourinho's side with his fourth goal of the season before Marcus Rashford equalised for United - only for substitute Camilo Zuniga to re-establish Watford's lead.
Captain Troy Deeney then completed the victory after he tucked away an injury-time penalty.
"We have shown that we can fight against anyone," Mazzarri added. "But the real step forward I want to see is how we will behave against a club like Burnley.
"I haven't been here long and I need to verify the progress of this team, and we will see on Monday night what the progress is and make more evaluations from there."