St Kilda coach Grant Thomas says the Saints have not given up on their premiership aspirations, despite the shock loss of star midfielder Lenny Hayes for the rest of the AFL season.

Hayes, whom Thomas said was in "rare form" and would have been leading the club's best and fairest count at this stage, found out on Monday he had ruptured his anterior cruciate ligament (ACL).

It came as a stunning blow, given he sustained the injury in the opening quarter of Sunday's match against the Kangaroos at Telstra Dome, but had no suspicion it was serious and continued playing.

"I got up a bit gingerly, but didn't think anything was too bad," Hayes said.

"I played out the game, I was a little bit sore at halftime, but I just thought it was from the knock that I got.

"I didn't think it was too bad and then sort of woke up last night in a fair bit of pain, I was a little bit stiff, but again I just thought it was from the knock.

"Then they decided to get it examined today and found out that I'd torn the ligament."

St Kilda medical staff examined the knee at halftime, but because it was stable they gave Hayes the all-clear to continue playing without realising the seriousness of his injury.

Hayes said he was "bitterly disappointed" and that it was a harder pill to swallow because the diagnosis was so unexpected.

"If I came off on a stretcher it's probably easier to accept because there's been a major incident ... it's a pretty rare case, the rest of the knee's pretty stable," he said.

Thomas said while the latest in a string of injury blows to star Saints would make things tougher for the club, they could still win the premiership.

"Of course we can," Thomas said.

"Whilst a team's made up of personnel and Lenny's really high up in the order within our team, sometimes challenges like these are the sort of things that ignite other careers and other opportunities.

"It's a very even year - obviously there's two or three sides out the front at the moment and then I think there's a big pack.

"But first things first, we've got to make sure we win enough games to get into the finals and once we do that I'm very certain that we can be very competitive again.

"While someone of Lenny's quality enhances your chances, I don't think they're necessarily negated by him not being there."

With Aaron Hamill (knee), Justin Koschitzke (skull) and Xavier Clarke (hip) already on the injury list for significant periods, and the large number of injuries experienced by the club last season, Thomas said the Saints knew how to fight through such setbacks.

"We've had a fair bit of adversity in relation to injury in recent history and we've had to all adapt," he said.

"Lenny's done his part in the last couple of years and now it's the turn for others to take the step that's required.

"I think Lenny would be far more disappointed than he currently is if he felt the playing group thought the season was over."

Thomas said Hayes' standing within the club meant his forced absence could inspire greater performances from his teammates.

Hayes, who will have surgery on Wednesday, will then spend some time with his family in Sydney and is hopeful of returning to the AFL for round one next season.

Meanwhile, he was on Monday charged with striking `Roos skipper Adam Simpson and offered a one-match suspension, although that becomes little more than a formality, given his injury.

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