Bulldogs officials had initially feared the worst when Darcy landed awkwardly from a marking contest in the 71-point thrashing of St Kilda on Sunday night.
The 21-year-old key forward fell to the turf clutching his left knee and was substituted out of the match within minutes.
Scans confirmed Darcy avoided damage to his ACL, but he suffered a significant injury that will require surgery.
"Scans have cleared the ACL of damage but confirmed that he has suffered a significant knee injury, with an impaction fracture at the front of his knee and associated ligament damage at the back of his knee," Bulldogs head of sports medicine Chris Bell said.
In a statement, the Bulldogs said they will have a clearer picture of Darcy's recovery timeline in coming weeks.
It is just less than 20 years to the day since Darcy's father - 226-game Bulldogs great Luke - suffered the first of two serious knee injuries that sidelined him for most of 2005-06.
Coach Luke Beveridge intends to get creative to replace 205cm target Darcy in the forward line as the eighth-placed Bulldogs (3-3) chase a finals berth.
Beveridge will turn to his "role-player options" to fill the void next to Aaron Naughton, and hinted he could also use a smaller line-up, starting with the round-seven meeting with GWS in Canberra.
"Buku (Khamis) can play either end of the ground, as James (O'Donnell) can," Beveridge said.
"We think long-term James is probably more of a key back for us.
"It's not in the Plan A approach to the game to play Rory (Lobb) forward at the start of a game, but it is an option.
"He'll do anything for the team - he's a great team man like that.
"And the other option is to just play a bit smaller at times."
Beveridge's short-term fix won't include troubled star Jamarra Ugle-Hagan, who remains unavailable for selection as he deals with personal issues.
The 23-year-old was the Bulldogs' top goal-kicker with 43 majors last year - ahead of Darcy with 38 - but has not played at all this season.
Beveridge said last week Ugle-Hagan's playing future is in the hands of the AFL, and on Sunday night he insisted Darcy's injury hadn't raised the chances of a recall for the former No.1 draft pick.
"That's obviously not going to happen in the immediate term, so it's probably not worth discussing at the moment," Beveridge said.
"As far as any replacements go, we really can't consider 'Marra'."
One major positive in attack against St Kilda for Beveridge was Naughton's improved form after quarter-time.
With Darcy subbed out before the first break, Naughton kicked 3.3 from 12 disposals as the most dangerous forward on the ground.
Naughton managed just five goals in his first four games this season, but has kicked six in his past two.
"He really stood up and put his leadership hat in the ring," Beveridge said.
"He probably missed some chances, too, to really get some spoils, but I'm really happy with the way he responded after quarter-time.
"Early in the game he probably wasn't quite where we needed him to be, even with Sam there, so it was a pretty important role he played tonight."
Darcy kicked 38 goals in a breakout campaign last year and added 14 as the Bulldogs' most prolific forward through their first five games this season.
Widely regarded as a generational talent, Darcy signed a four-year contract extension with the Bulldogs last December, which ties him to the club until the end of 2029.
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