Lynch, 36, is the oldest player in First Utility Super League and his absence is a major blow to Castleford ahead of Saturday’s Ladbrokes Challenge Cup sixth round tie at home to Salford Red Devils.
“He has had a syndesmosis and a break of his fibula,” coach Daryl Powell confirmed.
“It is not a great injury. He had an operation [on Wednesday] and he’ll be out for 10-12 weeks.
“He will be back this year.”
Powell added: “He has been an ever-present, such a pivotal figure for us.
“On the back of Michael Shenton being out for a significant part of the season, this is a bit of a blow.
“We know there’s a number of players in our middle unit who are going to need to step up and I look forward to that this week.”
Larne Patrick, Junior Moors, Luke Dorn and Jake Webster have all recovered from injury and are back in contention for Saturday’s tie.