GEELONG onballer Brent Prismall is "almost certain" to have ruptured his anterior cruciate ligament requiring a full knee reconstruction.

Prismall, 22, left the MCG during the first quarter of Sunday's qualifying final win against St Kilda on a stretcher after his right knee buckled underneath him in the left pocket at the Punt Road end.

He crashed to the turf after his leg gave way in what appeared an innocuous movement as he pushed to evade St Kilda's Robert Harvey, but coach Mark Thompson feared the worst after the match.

"He's okay; he's a little bit disappointed. We're not certain but we're almost certain that the signs are that he may have done his anterior cruciate ligament," Thompson said.

"The early signs are that he has, but we've just got to wait until we scan him to confirm it 100 per cent.

"We were really pleased that he's in our best team in the first final and it was up to him whether he stayed there. He's a terrific young kid and it's just a bit of a tragedy really. You don't like to see that happen to any player from any team."

The injury will likely see Prismall sidelined for around 12 months, which is a cruel blow for the midfielder who was playing in his first final after struggling to cement a spot in the talent-laden Cats side.

Premiership players James Kelly and David Wojcinski will push for inclusion in the preliminary final side, but Thompson said it was unfair to consider them ready-made replacements.

"We would love to have Kel and Wojcinski in the side, but I'd also love to have a really healthy Brent Prismall," he said.

"It's really sad that he got injured and to say that we've got ready-made replacements shows a bit of disrespect for him." 

Prismall was playing in just his 25th senior game since making his debut in round five, 2006 after the Cats picked him 32nd overall in the 2004 NAB AFL Draft.