Jarman Impey's AFL season could be over after the Hawthorn defender hyper-extended his right knee in the big win over Geelong.
Impey's leg collapsed in a marking contest and he needed help from two trainers as he limped from the MCG.
Despite losing the backman early in the second quarter, the Hawks held off their fierce rivals for a 24-point win.
It is their third-straight win and the Hawks are a game outside the top eight.
Coach Alastair Clarkson said they were unsure about the extent of the injury and compared it to the scenario around GWS star Stephen Coniglio a week ago.
The Giants also feared Coniglio would need a knee reconstruction, but despite still undergoing an operation, he might return for the finals.
Clarkson said Impey has lateral ligament damage and that could end his season.
The bigger worry is if there is damage to the anterior cruciate ligament, meaning a knee reconstruction and a much longer recovery.
"We're at the mercy of the scan," Clarkson said.
"If he's torn a lateral ligament, that's enough to miss a fair spell of footy.
"It's hard to say, but we're obviously just more concerned with whether or not it's going to be a 12-monther, than what it's going to do ... right now."
While the Hawks have their fingers crossed on Impey, otherwise they are delighted after upsetting the ladder leaders.
Asked about the significance of the win, Clarkson pointed to younger players such as Mitch Lewis and Tim O'Brien, who kicked three goals apiece, James Worpel and Conor Nash.
"These guys are still trying to find their way," he said.
"It just gives them belief that they can contribute in the role that they play.
"Geelong are a big opponent, a big day and when we play a side like that, we need 22 contributors.
"For the side generically ... beating a side like the Cats just gives us a bit of belief that we're not too far away."
Australian Associated Press