He conferred with coach Neil Craig after training on Wednesday, and again briefly yesterday, and they agreed that the shoulder was "popping out" too easily to warrant selection in today's first elimination final against Collingwood at AAMI Stadium.

"It got to the stage where it [the shoulder] was becoming looser and looser," Craig said. "There were times when it didn't come out, but it was starting to move on just some everyday living activities, if you like … opening the fridge or cleaning the teeth or going to the toilet.

"To Jason's credit, he was very honest with himself and obviously the medical people and myself. It got to a stage where it wasn't conducive to Jason being able to perform to a level he had to be able to do to be selected.

"The timing is now right for Jason to get that operated on. It will be a six-month rehab - he'll be able to train during that time, obviously - but it'll be six months before he'll be able to play again."

It was thought that if Porplyzia did not play Adelaide would recall Brent Reilly, who has shown quality form in the SANFL despite an injury-plagued season. But the Crows opted to bring in a third tall player in Brad Moran, who can play as a ruckman, or key positions up forward or in defence.

"We are keen to play Kurt Tippett, Brad Moran and Ivan Maric, all young future big men of our footy club," Craig said. "I think finals dictate some taller-type guys, particularly with the way we think the weather will pan out."

Meanwhile, Collingwood have admitted agonising over the decision to drop experienced defender Shane Wakelin.

The dual grand final backman, who was overlooked in favour of Simon Prestigiacomo, was unlucky to miss selection, Magpies assistant coach Brad Scott said.

"[It was] extremely close," Scott said. "It's fair to say there was a lot of debate at match committee about it. It's a very tough decision."