Melbourne's score of 16.11 (107) marked the second time Fremantle have conceded 100 points or more this season, after Geelong piled on 147 points in round one.
The Dockers only conceded triple-figure scores on three occasions across all of last season.
When Fremantle last made the finals - in 2022 - the highest score they conceded all season was 91 points.
Longmuir is taking part of the blame for his team's backward step in defence, hinting the outside criticism urging them to be more attacking may have played a role.
"We get criticised for being a defensive team at times, and I think our fans would have liked us to have been a little bit more defensive on the weekend and keep a low scoring team to under 100 points," Longmuir told reporters on Tuesday.
"Twice this year we've conceded 100 points. Last year, it was three times across the year.
"We've lost our defensive identity at times, and we need to make sure we get a balance right with what we're coaching."
The lack of forward-line pressure against Melbourne was evident, but Longmuir said it was a whole-team mentality that hurt them.
"It's not just the forwards. We've been inconsistent with our team defence all year," Longmuir said.
"We've had some moments and kept some teams to low scores, but it hasn't looked the way we wanted it to look as consistently as we could.
"Even the week before against Richmond we didn't defend well in the first half of the game.
"So we need to become more consistent across the team. It's called team defence for a reason."
Fremantle boast one of the most talented young squads in the AFL.
Longmuir is happy with the personnel at his disposal, but believes the mindset needs addressing.
"I would say it's mindset, because there's been times when we've been able to do it and lock teams away and play the game in our front half," Longmuir said.
"I would say (it's about) consistent mindset, consistently trying to hunt the opposition, rather than accept being hunted.
"There's patches in games where we're able to execute well.
"We just haven't had that ruthless mindset to be able to lock in for four quarters of our footy.
"We've got to expect the opposition's best as well, which maybe we didn't on the weekend."
Longmuir said defender Heath Chapman (illness) was on track to return for Friday night's clash with Adelaide in Perth, but Jaeger O'Meara (hamstring) appears set to miss.
Ruckman Luke Jackson is still one week away from resuming from his hamstring injury.
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