Geelong coach Mark Thompson says the loss of his talented young midfielder Brent Prismall took the gloss off his team's ten-goal rout of St Kilda in Sunday's qualifying final at the MCG.

The Cats took a giant step towards winning back-to-back flags for the first time since 1952 by thrashing the outclassed Saints by 58 points, despite only having three fit players on the interchange bench for most of the match after Prismall suffered a suspected serious knee injury midway through the first term.

The win gives the Cats a week off and an MCG preliminary final against either the Bulldogs or Sydney in a fortnight's time - ahead of what appears an inevitable grand final showdown against Hawthorn.

But Thompson's only thoughts on Sunday night were for Prismall, whose knee simply buckled underneath him as he was being chased by St Kilda veteran Robert Harvey during the early stages of Sunday's match.

"We are almost certain he has done his ACL (anterior cruciate ligament)," Thompson said of an injury that usually means 12 months on the sidelines.

Prismall, an emergency in last year's premiership winning team, has long struggled for senior opportunities at Geelong despite widely being regarded as a certain starter in just about any other AFL team.

But this year the talented 22-year-old has all but doubled his games' tally from the past two seasons to 25 and was finally on the verge of establishing himself in the side.

"He was in our best team for our first final and it was up to him whether he stayed there but he is a terrific young kid and it's a bit of a tragedy really because you really don't want to see that happen to any player in any team," Thompson said.

Not even the fact the Cats already have obvious replacements for Prismall in James Kelly and David Wojcinski could cheer up Thompson post-match after he learned of Prismall's fate.

"We would love to have Kelly and Wojcinski in the side but we also love to have a healthy Brent Prismall and to say we have ready-made replacements shows a bit of disrespect for Prismall," he said.

Star forward Paul Chapman also sent a scare through the Cats by complaining of both a bad corky and hamstring soreness late in the game but he is expected to recover given the Cats don't play for a fortnight.

Thompson was delighted with his team's performance despite saying his team - which has now 41 of its past 43 matches - was below its best in the slippery conditions.

"As a team we didn't have any super players on the day but it was a pretty well balanced effort," he said.

"We have been in good form but today the conditions didn't help us play our best footy but in saying that to kick nine goals in one quarter (the third) on a wet track is a good effort."

Thompson however heaped special praise on first year defender Harry Taylor, who kept Saints' skipper Nick Riewoldt to just nine possessions and one goal.

"I know he (Taylor) has had some personal criticism and he has been seen as the weakness in Geelong but we have been really supportive of him and he played a terrific game individually and he got some amazing help from his teammates."