The Holland international defender, who joined the Hornets from Newcastle only last month, fell awkwardly in his side's 3-1 win against Manchester United on Sunday
Janmaat, who set up Etienne Capoue's goal in the victory at Vicarage Road, was replaced after 51 minutes, and Watford manager Walter Mazzarri has confirmed the 27-year-old now faces a lengthy spell on the sidelines
"We really didn't need this now as it is quite a bad injury," Mazzarri said
"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 United, but he is now expected to be replaced by Nordin Amrabat for their match at Burnley on Monday night
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."
Source : PA
Source: PA