Darcy attempted to push off his left leg in the forward pocket at GMHBA Stadium on Friday night and immediately fell to the ground holding the knee.
The 22-year-old lay on the ground for a few moments and punched the turf in frustration before being helped to his feet by trainers.
In a positive sign, he walked off field without assistance, but was taken directly down to the Dogs' change room and was ruled out for the rest of the game.
He will have scans on Saturday to determine the extent of the damage.
"It's obviously a pretty serious knee injury," Bulldogs football manager Matthew Egan told Fox Footy at halftime.
"The mechanism didn't look great, but we just can't confirm everything.
"Unfortunately I've been here before where I've said it's a serious knee injury and we think ACL, and it wasn't.
"So we need to be positive and just wait for the scans."
Geelong dominated early and led by 38 points when Darcy was hurt early in the second quarter.
The margin ballooned to 57 points by halftime, with Cats speared Jeremy Cameron having kicked seven goals to that point.
There were fears Darcy had sustained a torn anterior cruciate ligament when he hurt the same knee in a win over St Kilda in April last year.
But he avoided the need for surgery and missed only six games with an impaction fracture and associated ligament damage.
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.
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