The controversial new substitute rule is going to test the nerves of St Kilda coach Ross Lyon.

While Geelong had no choice but to use substitute Darren Milburn early in Friday night's AFL game, Lyon kept second-guessing himself before bringing on Brett Peake in the third term.

There was plenty of focus on the substitute system during Thursday night's season opener between Carlton and Richmond and it was also a feature of Geelong's one-point win.

The Cats had to bring on Milburn when acting captain Joel Selwood was concussed in a sickening collision with Saints midfielder Farren Ray, who is now on report.

The AFL's tightening of concussion guidelines, announced just before the start of the season, meant Selwood had to be subbed out of the game.

But the Saints had more discretion with Peake.

He replaced Andrew McQualter, who had some tightening in a calf muscle.

"Gee, it puts some pressure on you, making the call," Lyon said.

"It is Russian Roulette - I'm quite conservative.

"You make the call and anything can happen, it's going to be interesting."

Once a player is subbed out of the game, he cannot return.

Lyon said they were unsure whether to bring off McQualter or Stephen Milne, who was tiring.

"We spoke about it probably three or four times before we subbed him," Lyon said.

"We went to do it and I thought the risk is enormous.

"We went to sub and then it looked like Milne was gone ... it's interesting, all part of the business."

Geelong coach Chris Scott did not want to add to the debate over the substitute system, which is designed to restrict teams' use of the interchange bench.

He said it would take some time to work out what effect the sub system is having.

Brought to you by AAP © 2024 AAP