Mills' hamstring injury creates a dilemma for the Swans coach and his football department, with fellow backman Robbie Fox and unlucky midfielder Taylor Adams two obvious candidates.
Mills was hurt at training last week and Longmire said on Monday he had not been ruled out should Sydney win through to the grand final.
Adams was dropped for Sydney's qualifying final win over GWS, while Fox is the obvious call if the Swans want another defender to replace Mills.
"I'm not sure yet. Obviously we weren't sure until we realised who we're going to play, now we've got that in front of us," Longmire said on Monday.
"We're talking through those things at the moment.
"Hopefully there's a couple of options there. Is it a like-for-like type? We know we can't replace his leadership abilities.
"We've got some options there that we're looking at at the moment. Nothing's locked in."
Robbie Fox is an option to replace Callum Mills for Friday's preliminary final. (Bianca De Marchi/AAP PHOTOS)
The Swans must overcome an eight-game losing streak against Port that goes back to 2016.
Most recently the Power humiliated them by 112 points on August 3, taking a 71-point lead before the minor premiers posted their first score.
But Longmire leaned into the narrative that the "season starts again" in the finals, saying they won't waste time reviewing their round-21 disaster.
"It doesn't really help you. We know we weren't good enough and Port Adelaide were terrific," Longmire said.
"That's where it's at ... this week's a new week."
"It's just about playing well this week. They're a very good football team. They have been for a long time.
"They've been so consistent. They've been coached well. They're a great playing unit, and so they've always been hard to compete against and they certainly played well against us last time."
Sydney have no desire to revisit the thrashing they copped from Port Adelaide last month. (Michael Errey/AAP PHOTOS)
Longmire said resisting Port Adelaide's pressure around the contest would be crucial after watching their semi-final win over Hawthorn.
"It was red-hot early on in the game, and it stayed that way for the entire game," he said.
"It was a fierce contest and Port Adelaide were able to apply enormous pressure on the opposition.
"They were able to withstand the heat that Hawthorn were able to apply and they were able to stand up in the moments, and to their great credit.
"That's the thing that really stood out."
Tensions boiled over after the final siren at Adelaide Oval and Port were fined $20,000 for taunts made by coach Ken Hinkley to Hawthorn players.
"The only thing I will say is, coaches are front and centre of a lot of things, and it's a pretty tough job," Longmire said.
"Kenny's done a fantastic job with that football team."
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