Watford could be without top scorer Joao Pedro for up to two months due to ankle ligament damage.
Although he initially attempted to continue, the 21-year-old had to be replaced before half-time.
"He's going to be out for about six to eight weeks. It's a big blow," said Watford boss Slaven Bilic.
"None of the ligaments are completely ruptured, but they all have damage.
"I don't have to tell anybody what Joao means to this team. He's exceptional, he's someone you can build a team around and we will miss him."
Watford, who are in the Championship play-off positions going into Friday's game at Swansea, already have a sizeable injury list.