Walter Mazzarri has rued "a bad injury" suffered by in-form defender Daryl Janmaat that could rule him out for a lengthy period.
The Watford right-back had to be replaced after 51 minutes of the club's 3-1 win over Manchester United last weekend with a shoulder problem, having earlier set up the opening goal for Etienne Capoue.
Despite avoiding a dislocated shoulder, reports suggest he is set to miss around six weeks of action if it is confirmed he has suffered a hairline fracture.
"We really didn't need this now as it is quite a bad injury," Mazzarri said at his media conference ahead of Monday's trip to play Burnley.
"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.
"We need to evaluate the results of tests and find out if there is a fracture before we can put a timescale on his injury, but it is a blow - Daryl has done well since he joined us.
"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."
Mazzarri also reported that defender Jose Holebas (groin) is a doubt for the Burnley clash, which the former Inter and Napoli boss sees as a big test of his team's mentality.
He said: "We have shown that we can fight against anyone, but the real step forward I want to see is how we will behave against a club like Burnley."