Within six months, Cameron McInnes has gone from inspirational St George Illawarra leader during the trying end to Paul McGregor's time as coach, to shock Sharks signing.

And now with the ink barely dry on his 2022 Cronulla contract barely dry, his time at the Dragons is over in an instant.

Training on Friday, McInnes suffered an anterior cruciate ligament tendon injury, meaning his hopes of one last push for finals action with the Dragons was over in an instant.

Instead, 2021 will offer the 27-year-old, who switched between hooker and lock last year, only has surgery and rehabilitation on the agenda.

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It's a blow for St George Illawarra, who will look to fast-track the Andrew McCullough deal, after he was released by the Broncos on Thursday.

What seemed as a bizarre move to allow McInnes to move on while pursuing McCullough, who has had an injury hampered career in Brisbane and Newcastle, now seems a blessing for coach Anthony Griffin.

McInnes had welcomed McCullough's imminent arrival when addressing his move to Cronulla, but the 'experience and toughness' he spoke about might be more valuable than ever now.

"I want the club to be successful on the field so if Andrew McCullough was to come I think he'd add a lot to the team with his experience and toughness," he said.