The first heartbreak of AFL grand final day has struck early, with Geelong's Max Holmes to miss the game with a hamstring injury.

Mark O'Connor will come in to the 22, and Brandan Parfitt will be the medi-sub.

The big decision was reportedly made on Friday afternoon, despite Holmes moving well during a fitness test on his injured hamstring that morning.

The 20-year-old wingman, who suffered the injury during the preliminary final against Brisbane, did not take part in the Cats' light training run at the MCG on Friday afternoon.

He spoke to club staff on the boundary line as his teammates completed light work ahead of Saturday's season decider against Sydney.

"Mark O'Connor comes in and Brandan Parfitt will be our sub," Cats coach Chris Scott said.

"It's desperately unlucky for Max because he was so close. This was one of those calls where, in my mind, he is probably 98 per cent there but we just needed certainty for the team, it's really hard on him.

"He probably could have done it but this was the right decision under the circumstances and Brandan and Mark have been good for us this year.

"We are just going to go with the fit guys."

Irishman O'Connor has been the medi-sub in the Cats' two previous finals.

On Friday, Scott said O'Connor was "probably" the man to come in if Holmes was ruled out.

"There are three guys over the last couple of weeks in our squad, in particular, that have been desperately unlucky," Scott said.

"Mark's been one of them, Sam Menegola is another and Brandan Parfitt is probably the other.

"There's another group of players beyond them but if you had have asked me 12 weeks ago, I would've said that those three guys are in our team for sure.

"I feel desperately for them but if Max doesn't come up it will be one of those guys that comes in."

Sydney have also had injury concerns during the week, but coach John Longmire insists Sam Reid (adductor) and Justin McInerney (ankle) will play.

Reid was substituted out of the preliminary final win over Collingwood but McInerney managed to finish the game.

"(Reid) got some scans on Monday and that was really encouraging, so we were confident from that point," Longmire said.

"He did some training on Thursday and did exactly what he needed to do.

"Our medical staff have been terrific, we were always confident that (McInerney) would be right.

"He finished off the game strongly last week so he's right to go."

AAP