Setanta Ó hAilpín ruptured his anterior cruciate ligament when playing for his new Aussie Rules club Greater Western Sydney yesterday.
The big Corkman sustained the injury during the third quarter of his side's 67-point Premiership defeat to his former employers Carlton at Etihad Stadium.
Subsequent scans confirmed the damage to his left knee and Ó hAilpín will meet surgeons later this week to decide on the best course of action for the former Hurler of the Year.
It was a nightmare end to what had been a promising debut for the 29-year-old. He kicked two goals for the Giants and had an energetic presence in attack, after missing their first five Premiership games with a calf injury.
He is expected to miss the rest of the AFL season, particularly if he goes down the more traditional reconstruction route. LARS surgery, which could fast track his recovery, is also an option.
On his Twitter page, Ó hAilpín said: "I would like to thank everyone for their support in trying times. All messages are greatly appreciated."