The 23-year-old will undergo an operation after damaging his cruciate ligament for the second time in four years.
The injury blow came in the Tigers’ 2-1 victory over Southampton almost a fortnight ago.
“Everybody at the club wishes Will well for a full recovery ahead of what is going to be a long road back to action,” head coach Mike Phelan told the club’s official website.
“To lose a player of Will’s quality and potential for such a long period of time is a big blow, but I’m sure his determination and mind set will see him through the tough journey ahead.”
The striker joined Hull from Manchester United in August, signing a three-year deal.