Mark Edmondson, one of Castleford's key signings for their return to Super League next year, has been forced to retire at 28 after failing a club medical.
The former St Helens forward agreed a 12-month contract after two injury-plagued seasons with the Sydney Roosters and Salford, where he suffered serious shoulder damage.
"This is disappointing for the club, but it is a much bigger blow for Mark," said the Tigers' football manager, Mick Robinson. "He is a talented player and ordinarily would be looking forward to many more years playing the game he loves."
Castleford will look for a replacement.
Widnes have denied making a move for the Wales rugby union centre Gavin Henson, but have not ruled out doing so in future.
"We have not been in contact with him or his agent," said the club's football manager, Pat Cluskey. "However, should we get a Super League licence for 2009, Gavin Henson is the type of player we would be looking to attract.
"We would be looking to get back to the days when we attracted the world's best players. We used to do that and plenty of them were from Wales."
Henson, who recently broke his hand, faces a charge of disorderly behaviour on a train. It is not thought inconceivable that he might fancy a fresh start in a different game.
Widnes are interested in recruiting Tommy Gallagher, the Hull KR forward who spent part of last season on loan to Batley. Gallagher, who played for Widnes in 2004, can play anywhere in the pack. A number of forwards left when Widnes's future was uncertain last month.
Bradford are giving a second chance as a professional to Jode Sheriffe, younger brother of Harlequins' Rikki, who failed to make the grade with Halifax last season.