The Uruguay international was stretchered off during Tuesday's 3-2 loss to Wolverhampton Wanderers in the Championship.
Tigers boss Leonid Slutsky told the club website: "We have lost Abel for six months, which is very, very disappointing. For me, Abel is one of the best forwards in the Championship and is a huge player for us.
"It is also a big blow for Abel himself, because he had made a very good start to the season and was playing very well for the team.
"We wish him all the very best in his recovery and look forward to seeing him back out on the pitch as soon as possible."
Hernandez is out of contract at the end of the season.