Amartey was subbed out of the contest in the opening half after limping off Adelaide Oval and clutching at his thigh, with early signs suggesting the impressive forward would face an extended sideline stint.
The ailment dampened a strong start to the match for Amartey, who had booted two early goals to extend his impressive run of form in 2023.
The Beaumaris product is now set to spend at least two months out of action, adding to a tough run with injuries for the Swans right as Amartey was finding form.
"Yeah (it's) unfortunate for Joel, and us. He's had a wonderful start to this year," Longmire told AFL360 on Monday.
"He's started to really deliver on some of the promise that we knew he had. Hew was really starting to get confident and start to use his body in the way we know. He's a great power athlete.
"It was terrible for him to turn and do his hamstring in the way he did. Unfortunately, there's some tendon damage, so it'll be an eight to 10 week (setback).
"We won't get to see him at the earliest until after the bye. It's very disappointing for him."
The Swans will remain without McCartin brothers Paddy and Tom for their Round 6 bout with Geelong due to concussion, with the pair not deemed fit to play after suffering head knocks in their Round 4 loss to Port Adelaide.
Having battled with a history of head trauma in the past, Paddy McCartin's recovery will see the club place no timeline on his return, with his health and well-being a priority as he remains on the mend.
Longmire revealed McCartin's symptoms are improving each day, but a return date was not set.
"Paddy is in training every day," Longmire said. "He's not doing any training as such, but he's been here every day and we've been catching up and chatting.
"It's just a matter of making sure he's okay and hopefully starting to feel better day by day, which he is.
"That's the main part that we're mindful of.
"(The symptoms) are starting to (pass). He's a lot better today than what he was four to five days ago."
The Round 6 clash will see last year's grand finalists go head-to-head, with Geelong to also unveil their premiership flag prior to their first game at Kardinia Park for the season.