Salford Red Devils winger Mason Caton-Brown will be out for up to 12 weeks with a serious leg injury.
Caton-Brown, 22, will undergo surgery following a tackle by Huddersfield's Danny Brough in Monday's 26-24 win.
The tackle was cited, but after viewing the incident, the RFL decided not to take any action towards Brough.
"We're not happy about it. We've asked the RFL to look at the incident. They have come back telling us it's legal," head coach Ian Watson said.
"From our point of view, we're really disappointed with what they've come back with as we've lost a good player."
Caton-Brown was making his first appearance of the season for Salford since coming back from injury, but now faces another lengthy spell out.
"He has only just come back from an injury, he's been out for a long time," Watson told BBC Radio Manchester, "He's a really nice kid and in your first game back to go down like that when you're playing so well is a big loss."
Salford moved up to seventh in Super League following Monday's win, which was marred when fighting broke out among a section of Salford fans at full-time.