Watford removed Liverpool’s cloak of invincibility by inflicting on Jurgen Klopp’s men their first league defeat of the season, but Deulofeu missed a chunk of the game through injury.
The 25-year-old Spaniard went down clutching his right knee shortly before halftime following a tussle with Virgil van Dijk and was replaced by Roberto Pereyra after receiving extensive treatment on the pitch.
Deulofeu has made 28 appearances in the Premier League for Watford this season, scoring four goals and supplying five assists for his teammates.
Following scans conducted on the former Barcelona and AC Milan forward, Watford manager Nigel Pearson is not expecting Deulofeu to return before the end of the season.
"Unfortunately for us the scan results really show he is probably going to be out for the rest of the season," Pearson told Sky Sports News.
"The initial scan results show that he has got an ACL (injury), which is a really bad thing for us because he has been such an important player for us.
"He's a really good lad as well. I think it is upsetting for the players when one of your colleagues gets an injury like that during a game.
"Certainly for us he will be a miss, but having said that Maxi Perreira went in yesterday and I thought he did very well when he went on
"But for sure Gerry is going to be a miss for us. But we'll have to cope. It's important we look forward with a positive mentality but it's certainly not good news for us for sure."
