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 hobbled off the ground and was taken directly to the Dogs' dressing room for further assessment.
He was replaced by second-game substitute Cooper Hynes.
Broadcast vision in the second quarter showed Darcy's upset family in the dressing room.
His father, 226-game Bulldogs champion Luke, missed almost two full seasons in 2005-06 with serious knee injuries.
Darcy had changed out of his playing kit when he returned to the Bulldogs' bench before half-time.
He 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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