The Swans were blown off the park in an 81-point thrashing against Geelong on Saturday, with Longmire later admitting they got nothing right.
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Longmire and his coaching staff picked Reid to play in the decider despite the 30-year-old being under an injury cloud all week after picking up a groin injury in the preliminary final.
The key forward was substituted out of the grand final early in the third term after hobbling off late in the second quarter with just four handballs to his name.
"We were obviously confident, that is why we picked him," Longmire said in his post-match press conference.
"But we made a mistake."
The costly move came in complete contrast to Geelong's decision not to pick Max Holmes, who coach Chris Scott said was only 98 per cent fit due to a hamstring issue.
Sydney backed Reid's importance to the point that young forward Logan McDonald was dropped to bring in Hayden McLean as a ruck option, allowing Reid to remain in attack.
But the 30-year-old failed to fire while his direct opponent, Geelong young gun Sam De Koning, starred.
De Koning unexpectedly went to Reid instead of Lance Franklin and dominated in the air, recording 12 disposals, five marks and six intercepts by half-time.
In comparison, Reid had just four touches, no goals and one mark before being subbed.
John Longmire concedes Swans were overawed
Longmire also conceded his players were mentally and physically overawed in a defeat that will be hard to digest.
"From the first bounce, the game looked as foreign as what we have played it this year," he said.
"There was no element that we got right.
"No aspect of the game looked like the way we wanted to play it ... from the selection of Sam to the way the game looked, we just didn't get it right."
Asked if his team was overwhelmed by the occasion, Longmire replied: "That is what it looked like.
"We just didn't have our heads in the game from the very start.
"When they were getting defensive stoppage goals in the first quarter that we had planned for and done well before - it was one of our great strengths and they were able to do it.
"We weren't able to think clearly or perform at that level that we should have.
"They had 11 (scoring) shots to one in the first quarter which wasn't a good start.
"And some of the things we started with, you look for our structural stuff, our intent in scramble, our shape - normally we have got a very distinct way and we will look back on this and reflect and it won't look anything like it."
Longmire said the Swans had let down themselves and their supporters.
"Everyone wants a black and white answer, it was this or it was that," he said.
"There's never one thing. It's always just a number of different things and we will look at the reasons why.
"But I am confident that our playing group, they're a really good group and we have had some wonderful achievements and taken some real steps forward as a footy team, most of our players would have improved this year.
"We have just buggered it up today."
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