Hull KR prop Adam Walker is facing a lengthy spell out of the game after damaging an ankle in his side's 25-12 win at Warrington Wolves.
The 23-year-old Scotland international missed only three matches in his first season with the Robins in 2013 and was the only specialist front rower to feature in all nine First Utility Super League and cup fixtures so far this year.
"Unfortunately, he's suffered a pretty serious ankle injury," Hull KR coach Craig Sandercock said. "We won't know exactly how long he'll be out until we get the scans back but the physio thinks it is pretty significant and that it will be substantial length of time.
"It's definitely disappointing because I think he's been one of our best players. Someone is going to have to step up now and do the job."
Walker's twin brother Jonny, a close season capture from Castleford Tigers, suffered a knee injury in his third match while fellow front row recruits Justin Poore and Michael Weyman have missed six matches between them so far through suspension.
With two other forwards, Jame Langley (hamstring) and Jordan Cox (shoulder), facing late fitness tests, Sandercock will be grateful for the availability of Poore, who has completed his second ban of the season, for Thursday's derby with Hull FC at the KC Lightstream Stadium.
"They are all touch and go and, being such a short turn-around, it's going to be a tough one," Sandercock said.
Full-back Greg Eden is poised for a return after gaining some match practice following his recovery from a broken toe by playing on a dual-registration for Championship One club Gateshead on Sunday.
"Everyone is back in the reckoning this week," Sandercock added. "We need to have a lot of contingency plans because there are a lot of boys who are very doubtful of playing."