Helguson was injured after an innocuous collision with a team-mate earlier this week and will be sidelined for a minimum of three months.
The Icelandic international was the focal point of the Hornets attack and his absence leaves manager Ray Lewington with a striking headache.
Lewington admitted everyone initially thought the injury was not serious with Helguson appearing to shrug off the effects of the knock.
But tests have confirmed the 26-year-old has torn a ligament, with Lewington cursing the misfortune.
"What's happened is quite unbelievable because it looked like nothing at the time," Lewington told the club's website.
"If anyone had seen the incident, they would have thought there was no problem at all. Probably like a lot of these type of injuries, initially you don't think it's too serious but then you get confronted with the news from your physio - after tests and a consultation - that the player's going to be out for a couple of months or more.
"By the time the physio got to the training pitch, he was all prepared to walk off himself but we put him on the stretcher, just as a precaution really.
"Now we're left with the news that we're looking at a tear in a ligament and between two to three months out for the lad."
