Castleford centre Jake Webster could face six months on the sidelines, depending on the results of a scan next week.
Webster, 29, hurt his shoulder in the defeat to Hull KR a fortnight ago, but later complained about a troublesome knee.
He will have another scan on the injury next week, and coach Ian Millward is unsure as to how long the former New Zealand international will be out for.
Millward said: “Jake Webster injured his shoulder in the Hull KR game and complained about his knee being sore. But after a couple of days his shoulder was OK but we had to take him for a scan because his knee had swelled up. He may need some non-surgical rehabilitation or he may need surgery.
“We’ll have to see [how long he’ll be out for] because we’ve got to do another scan. There’s a possibility that he could play for a month and see how it is but then he would have to have the operation six months down the track. So we’re in the lap of the Gods with that one.”