A broken collarbone sustained by hooker Rob Burrow took some of the gloss off Leeds' 32-12 win over St Helens in the fifth round of the Tetley's Challenge Cup.

The 31-year-old former England scrum-half went off after 64 minutes clutching his shoulder and Rhinos coach Brian McDermott confirmed news of the injury in the post-match press conference.

"He's broken his collarbone so that's not good," McDermott said. "I'm being told by the physio that it's 12 weeks but I always try to shave a couple of weeks off that."

It is a major blow for the Super League leaders, who have high hopes of claiming a first Challenge Cup triumph since 1999.

The durable Burrow, who made his 400th appearance for Leeds over Easter, is an ever-present this year and missed only one of his club's 33 matches in 2013 but he has already been ruled out of the quarter-finals on the weekend of June 7-8.

Papua New Guinea international Paul Aiton is Leeds' only other specialist hooker, but he is currently out of action with an unspecified illness and not expected back imminently meaning McDermott may have to use skipper Kevin Sinfield in the hooking role over the next two to three months.

"There are some things going around in my head at the moment but I'll have to give that some thought," McDermott said.

Full-back Zak Hardaker scored two first-half tries to put the Rhinos on their way to a thoroughly-deserved victory which left McDermott with mixed emotions.

St Helens struck back through Adam Swift and Josh Jones to keep the tie in the balance until Leeds scored late tries through Danny McGuire and Jamie Peacock, with Sinfield taking his goal tally to six from as many attempts.

"I'm not so excited about the performance," McDermott said. "Our desire in defence got us that game.

"I wouldn't say that has been the greatest performance from us, certainly not with the ball. The second half was very scrappy but I'd put that down to nerves.

"There was a huge amount resting on today. There had to be a loser of two teams who have a massive expectation.

"We certainly played nervous and I reckon Saints would admit to that as well.

"I'm absolutely ecstatic to be in the next round but we will have to be a lot better to progress further in this competition."

Although Hardaker took the scoring honours, the man-of-the-match award went to winger Ryan Hall, who set up one of the full-back's tries and caused Saints all sorts of problems with his powerful running out of defence.

"Some of the things Ryan Hall did defensively probably got us the game," McDermott said. "They asked a huge amount of questions of him and he answered them."

St Helens also suffered a major injury blow, with former Leeds prop Kyle Amor going off early in the second half with a knee problem, while a third-consecutive defeat threatens to undermine their flying start to the season.

Coach Nathan Brown admitted Hardaker's second try, with just four minutes left of the first half, proved crucial.

"Once again we started the first set with a dropped ball and we were defending our tryline for a fair period," Brown said.

"Defensively I thought we handled them pretty well until that try just before half-time, which was quite crucial.

"We let ourselves down a little bit with a flow of our attack. Our game management was just so-so."