Darcy was "managed" from the 15.21 (111) to 10.13 (73) win at Optus Stadium on Thursday, given the five-day break between games.
That decision came despite Darcy having played just two AFL games since recovering from a lengthy calf injury.
Cox hardly set the world alight during Darcy's stint on the sidelines, but the American came to life in front of a sellout crowd on Thursday night, tallying 10 disposals, six marks, 10 hitouts, two clearances and a crucial goal as Luke Jackson's back-up.
The 35-year-old's swagger was on display too, showing the ball to his opponent after taking one of his six marks.
Cox's powerful marking in the first half was crucial in ensuring Sydney's lead didn't blow out too much, with the former Magpie putting a strong case forward to be retained for next week's clash with the Power in Adelaide.
Dockers coach Justin Longmuir confirmed Darcy would have played ahead of Cox against Sydney if there had been a seven-day break leading into the game.
But given Cox performed so well against Sydney, Longmuir has a tough selection call to make.
"A little bit of horses for courses," Longmuir said of the ruck selection battle.
"I mean, everyone really needs to earn their spot in the side, don't they?
"We've got competition for spots, not just in the ruck, we've got competition for spots all over the ground.
"It's a healthy competition for spots. It keeps everyone on edge and preparing really well.
"Our rucks are probably a bit the same."
Longmuir was full of praise for Cox's efforts.
"I thought it was the best he's marked the footy, and I thought his ruck work was strong," Longmuir said.
The Dockers registered just 0.11 (11) in the first half before slamming through 15 goals across the final two quarters in a stunning display.
The win put Fremantle (15-2) two wins clear of the second-placed Swans (13-4).
Star Fremantle big man Luke Jackson (17 disposals, 26 hitouts, seven clearances) and Brodie Grundy (16 disposals, 29 hitouts, five clearances) broke even in their ruck battle.
Dockers spearhead Josh Treacy kicked 4.4 to help nullify a five-goal haul from Charlie Curnow.
Fremantle skipper Alex Pearce played an influential role in the win, producing a series of game-saving marks and intercepts to slow down Sydney's scoring.
"The skipper came out and led from the front," Longmuir said.
"He doesn't second-guess himself, does he? There's no situation he doesn't want to be in. He's led by example all year and he just continues to do it."
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