Veteran Castleford prop Andy Lynch suffered a broken leg in Sunday's 30-20 home defeat by St Helens, the club have confirmed.
The Tigers' long-serving front rower - at 36 the oldest player in Super League - was helped from the field six minutes from the end of the match after going down awkwardly in a tackle.
"Andy Lynch has a leg break to his fibula," said Castleford coach Daryl Powell. "It will be assessed over the next few days to see the severity of the injury and the time-scale out of the game but it will be a significant time."
Lynch said on Twitter: "Thanks for all the messages means a lot. Gutted is a understatement. Road to recovery starts now will be back in no time."
Lynch, who is in his second spell with his home-town club after stints at Bradford and Hull, became the oldest player in the top flight following the retirement of Jamie Peacock and Adrian Morley at the end of last season.
He signed a new contract for 2017 on the eve of the current season - his 18th in Super League - in February and has an option to play on in 2018.
Castleford were already without eight senior players for Sunday's game and gave a debut to 17-year-old prop Conor Fitzsimmons.