Darcy has undergone an arthroscope on his left knee and will miss the Dockers' tough round-one assignment against the Brisbane Lions on March 17.
They also face games against North Melbourne, Adelaide and Carlton in a testing opening month.
Fremantle on Tuesday revealed Darcy felt soreness in his left knee after their first pre-season trial game against West Coast.
The 25-year-old was then cleared to play against Port Adelaide last Friday night, but was taken out of that game at half-time and pulled up sore again.
A scan on Sunday revealed the need for an arthroscope, with the Dockers set to put a timeline on Darcy's recovery in the coming days.
"Following specialist advice it was agreed with Sean that the best course of action was to complete an arthroscope to remove some of the loose bodies that were identified from the scan to be causing the discomfort," Fremantle football manager Joe Brierty said.
"This procedure will enable Sean to get back to full fitness after an initial reloading period post the procedure."
It is another setback for Darcy, who underwent ankle surgery after his 2023 campaign was cut short at round 18.
The 98-game ruckman signed a six-year contract in December that ties him to the Dockers until the end of 2030.
Darcy's absence means Luke Jackson will have to step up in the No.1 ruck role, with Fremantle taking on Brisbane, North Melbourne, Adelaide and Carlton over the opening month of the season.
Emerging wingman Nathan O'Driscoll will also miss the early rounds following surgery to remove a blood clot superficial to his right knee.
"Nathan will be off-feet for a couple of days after the procedure, but it is expected that we will recover quickly from the procedure," Brierty said.Â
"Once he ticks off his load progressions, he should be able to transition back into football fairly quickly."
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