Darcy flew for a mark in the Bulldogs' forward line late in the first quarter at Marvel Stadium on Sunday night and hyperextended his left leg in an awkward landing.
The 21-year-old immediately grabbed at the knee and lay on the turf for a few moments before getting to his feet.
Darcy limped off the ground and was taken directly to the Dogs' dressing room for further assessment.
He was replaced by second-game substitute Cooper Hynes.
"The medical staff are pretty concerned at this stage and they're fearing the worst," Bulldogs football manager Matt Egan told the Seven Network at halftime.
"It looks like a possible ACL but we're just going to have to wait until tomorrow to get scans to confirm.
"At this stage we're fearing the worst, which is unfortunate because he's an incredible person and we'll obviously miss him.
"But until we get scans it's pretty hard to say too much."
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.
Broadcast vision during the second quarter showed Darcy's upset family, including his father, in the dressing room.
The 205cm forward had changed out of his playing kit when he returned to the Bulldogs' bench before half-time.
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.
