Darcy will miss a yet-to-be-determined period after injuring the same calf he strained in January.
The latest injury came during a rehab session on Saturday as Darcy was building his way through concussion protocols.
Darcy has a history of hamstring, calf, knee and ankle issues.
The 27-year-old was able to play 21 games in his best-and-fairest season in 2021, and follow it up with 21 games in 2022.
But he was restricted to 44 games across the next three campaigns as injuries started to take their toll.
Darcy signed a monster six-year deal at the end of 2023, tying him to Fremantle until 2030.
That deal has been increasingly put under the spotlight given Darcy's form and fitness woes, but Longmuir is backing the big man to play a significant role for years to come.
"I know Sean can have a big impact for us at AFL level. He's shown that - he's a best-and-fairest winner," Longmuir said ahead of Saturday night's clash with Carlton in Perth.
"Clearly it's been frustrating for us and him that he hasn't been able to get continuity, but I can see that changing, so I don't have any concerns long term."
Longmuir said Darcy has been putting on a brave face in the wake of his latest setback.
"He's acting himself around the club, and he's keen to get into his rehab and come back bigger and better," Longmuir said.
"It (the latest calf injury) is concerning, it's unfortunate, disappointing - whatever word you want to put to it.
"But we'll rehab it and move on. We need to get it right. There's still a lot of football to be played this year."
Darcy's injury means Collingwood premiership ruckman Mason Cox is now set to get an extended stint in the side.
Cox made his Dockers debut in Sunday's 56-point win over West Coast, and while he only tallied four disposals, two marks and 13 hitouts, he put himself in good positions and came close to taking a series of contested grabs.
Having Cox in the side allows No.1 ruckman Luke Jackson to spend important stints in the midfield and attack.
"We've liked that (two ruckman) policy because it gives Luke an opportunity to play in other positions," Longmuir said.
"It gives us more flexibility to move the magnets around in game as well.
"I was pretty happy with (Cox's) game on the weekend. He just didn't maximise his opportunities."
Veteran Jaeger O'Meara is scheduled to undergo surgery on Wednesday for the facial fractures he suffered at training last week, ruling him out for another month.
20°C