Darcy's left knee injury headlined the carnage of the most "traumatic" night of Luke Beveridge's coaching tenure.
Darcy went down clutching his troublesome left knee in the second quarter of the Bulldogs' 19.17 (131) to 8.8 (56) defeat to Geelong at GMHBA Stadium on Friday night.
Tom Liberatore (concussion), Rory Lobb and James O'Donnell (both hamstring) were also hurt in a horror outing as the Dogs (4-2) slipped to a second-straight loss.
The Bulldogs confirmed Lobb at least had been cleared of serious damage to his hamstring, with the key defender to be assessed ahead of Thursday's clash with Sydney, while there was no update on the severity of O'Donnell's injury.
Darcy fell to the ground under no contact, immediately sparking fears of the dreaded ACL tear, which was confirmed on Saturday.
Most players take around one year to return from a knee reconstruction.
"We're incredibly disappointed for Sam after scans confirmed he has ruptured his ACL," Bulldogs medical boss Chris Bell said in a statement."He's an extremely driven individual and we have no doubt he'll attack his rehabilitation strongly in the coming months."
The 22-year-old was fortunate to dodge that injury when he hurt the same knee in April last year, though an impaction fracture and associated ligament damage sidelined him for two months.
Beveridge had hoped for a similar outcome, but this time the Bulldogs will need to tackle the rest of their campaign without one of their biggest stars and focus on getting him back stronger.
Darcy's father - 226-game Bulldogs great Luke - suffered two serious knee injuries during his decorated career and was sidelined for most of 2005-06.
The first of those occurred at the same Kardinia Park venue almost 21 years ago to the day.
On Friday night, Geelong's coaches box and the 33,200-strong crowd fell eerily quiet when Darcy fell to the ground.
"I can't imagine their box being any more flat than ours when Sam Darcy went down," Cats coach Chris Scott said.
"You're not a real football person if your heart doesn't go out to individuals and the club in situations like that."
Beveridge forecast Jedd Busslinger and Ryan Gardiner would replace key defenders Lobb and O'Donnell for the Bulldogs' clash with the Swans next Thursday night.
Ruck star Tim English (knee) is no certainty to return, while Liberatore is sidelined again after he had only just returned from a hamstring injury.
