The Dockers were smashed in contested possessions (140-116), clearances (39-29), centre clearances (16-6) and inside-50s (68-44) during Saturday's 70-point loss to GWS - their biggest defeat under Longmuir.
Darcy is arguably Fremantle's most important player, and his return for Saturday night's clash with Essendon in Perth is crucial.
Not only does Darcy dominate in the ruck but his presence allows Luke Jackson to play predominantly in attack - a mix that has worked well for the Dockers this season.
"Our team balances a lot better with Sean in," Longmuir said.
"It allows Jacko to play the role he was playing before Sean got injured.
"I don't sit there and rank the importance of our players, but we've definitely missed him.
"The way he allows others to structure up, the way he allows others to play to their strengths, and the way he allows us to get a bit more proactive inside the stoppage when he's getting his hands on (the ball is important)."
With Essendon missing Sam Draper (hip), Darcy and Jackson have the chance to dominate Andrew Phillips in Saturday night's ruck battle.
Fremantle have also been boosted by the return of goalsneak Michael Frederick (ankle).
The Dockers looked every bit a premiership contender last month after stringing together four straight wins.
That stretch included a victory over the Swans at the SCG, beating Geelong in Perth, and a stunning triumph against Melbourne at the MCG.
But disappointing losses to Richmond and GWS immediately following the bye have dropped Fremantle to 13th spot with a 6-7 record, leaving their finals hopes in jeopardy.
In the wake of the humbling at GWS, Longmuir has reinforced the importance of his players sticking to their roles and working for the team.
"That's the main thing - a mindset change," he said.
"We need to make sure we get back to playing team footy, a connected brand, built on defence and contest, which wasn't evident last week.
"When you're not having an impact in your own role, you drift outside of it, maybe (go) chasing the footy.
"Sometimes when you value the wrong things in your game, when things aren't working for you, you can just go insular.
"That's what we did last week - we went insular."
The Bombers (8-5) are on a four-match winning run, and will take a major step towards locking away a finals berth if they can beat the Dockers at Optus Stadium.
Essendon will be bolstered by the return of gun midfielder Darcy Parish (calf) and defender Jake Kelly.
