The Rhinos played much of the match without Webb and McGuire who both limped out early on, while Clarkson was stretched off 10 minutes in to the second half of the fourth round clash, which Leeds won 30-20.
McDermott said: “Brent Webb has a dead leg and we hope the injury hasn’t caused too much damage. Ryan Hall had a similar injury against Featherstone in pre-season and we feared he would be out for weeks but was back the next week to play Crusaders.
“With Danny McGuire we are hoping it is a quad strain which could mean one to two weeks on the sideline however if it is a tear it could be five or six weeks.
“The good news on Chris Clarkson is that he has a badly bruised bone on his ankle and it is not a fracture. I think his pride is the deepest injury he has suffered as the boys were giving him some stick because he came off on a stretcher and there wasn’t even a break!”