The 20-year-old McDonald was brutally dropped for the grand final, with the Swans coach naming Hayden McLean - who hasn't featured in the AFL since round eight - as his replacement.
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"It's never easy, I've been on the end of that conversation before," Longmire said.
"It's tough you know, it's not easy on him. He's a young player with a great future for us and we just felt for this game we needed a bit more flexibility that Hayden gave us."
Longmire said stalwart Sam Reid and winger Justin McInerney both proved their fitness at Thursday training, having been under injury clouds that threatened their grand final hopes.
Reid, with Luke Parker the only players remaining from Sydney's 2012 flag-winning team, was substituted out of the preliminary final because of a groin injury, while McInerney suffered a sprained ankle.
"McInerney and Sam Reid got through it pretty well so we expect them to be ready to play on Saturday," he said.
If Reid fails to play, McDonald could earn a reprieve with the West Australian named as an emergency along with Braeden Campbell, Harry Cunningham and Will Gould.
"It's tough to go out of the team at any time but particularly for a grand final," Longmire added about McDonald.
Many fans took to social media after news of McDonald's axing filtered through, to convey their shock and disappointment for the Swans star.
Gutted for Logan McDonald. Such a stiff call and can only hope he gets a reprieve in the event Reid goes out. Tall forwards don’t grow on trees and feel this is a poor selection call from Sydney, irrespective of form. #AFLGF
— Jacob Landsmeer (@jlandsme_93) September 22, 2022
If u ever feel unwanted just Imagine being Logan McDonald 🪦
— Brianna Hayes (@gold104pointbri) September 22, 2022
How long before clubs start making enquiries about Logan McDonald, they'll be asking while he's still hurt from getting dropped for the GF.
GRAND FINAL TEAMS: Heartache for Swan, find out who's in - https://t.co/2odkNch1uq
— Steve W eighty 🏉🇷🇴 (@SteveW80) September 22, 2022
"It's a big move."
The panel looks at the named teams for the Grand Final, including the brutal call on young Swan Logan McDonald, who's been left out for Hayden McLean. #9MyRoomTelethon | Watch on @Channel9 pic.twitter.com/GwLllXdzc1
— Footy on Nine (@FootyonNine) September 22, 2022
Dear Pies. Logan McDonald must be gutted. I know you have your eyes on another key forward but pick Logan up instead.
— Pete Guley (@peteguley) September 22, 2022
Logan McDonald getting dropped for the final is such a shame, but can't help thinking it's the right call. #bloods
— Matthew Norman (@MattNorman_90) September 22, 2022
It didn’t bode well for Logan McDonald when he had little impact last week (and Swans weren’t keen to move the ball) after Sam Reid was subbed out. Tough call but has had a quiet month. Hayden McLean not as polished but can play second ruck if Reid breaks down in game. #AFLGF
— Andrew Wu (@wutube) September 22, 2022
Swans urged to trust their process against unchanged Cats
Geelong have named an unchanged side, including Max Holmes despite the wingman being subbed out of their preliminary final due to a hamstring injury.
The Cats, on a 15-game winning streak, enter the MCG finale as warm favourites.
But Longmire said his Swans, who have won their past nine games in a row, must believe in their system to cause an upset
"Just believe in what you do, the system," Longmire said of the message to his players.
"It's about turning up and about execution on game day.
"Don't use your energy up too early, save your energy for game day, and turn up and do what you do.
"They (Geelong) finished two games (wins) clear at the end of the home-and-away season and had a great win last week so they are clearly a very strong team.
"They have been fantastic this year but we look forward to that challenge."
"It is a big game and we just have to make sure we back in what we do."
Both clubs are featuring in Friday's grand final parade, which will have a markedly different look to past years.
Instead of travelling in slow-moving open-top cars through Melbourne's central business district, players and coaches from both clubs will be paraded along the Yarra River in a cavalcade of boats.
They will then greet fans at Yarra Park, outside the MCG.
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