Darcy sustained the injury in last week's loss to Collingwood and was one of five Fremantle players withdrawn from Justin Longmuir's plans for Round 19 when the Dockers face Sydney at Optus Stadium in what is deemed a must-win match to keep their finals hopes alive.
James Aish (quad), Bailey Banfield (lacerated knee), Brandon Walker (knee) and Nathan O'Driscoll (calf) have also made way due to injuries, with the Dockers confirming at selection this week that Mid-Season Draft recruit Ethan Stanley will make his debut against the Swans as the tactical sub.
While key recruit Luke Jackson is expected to take on the bulk of the ruck workload on Saturday night, the former Demon might need to prepare to do so for the final six weeks of the season if Darcy does require surgery on his hindered leg.
A decision won't be made until early next week however, with Fremantle football boss Peter Bell revealing to 6PR that the club is considering its options in regards to Darcy's recovery plan.
"It's reasonably serious. Sean's having consultations with some specialists and potentially may even have surgery on the ankle ligaments that he's damaged," Bell said.
"We're just working our way through that and we'll make a final decision early next week.
"It may be a little longer than we're all expecting, so we'll update accordingly early next week.
"... We have to make sure that we get (Darcy's) injury right, and if it is to be surgery or a longer term conservative management, then we have to cop our medicine there."
Darcy missed two games earlier this season due to a hamstring strain, with the club's mid-season bye helping his pathway through rehabilitation before returning in Round 15.
Fremantle host Sydney at Optus Stadium at 5:40pm local time on Saturday night.