The 26-year-old left the ground and was subsequently subbed from the game, with the Lions now eagerly awaiting scan results to determine the extent of the left ankle injury.
McStay's injury will provide yet another headache for Lions coach Chris Fagan, already dealing with the absence of Joe Daniher for up to six weeks after the key forward suffered a shoulder injury earlier in the week.
"When it rains, it pours," Fagan told media post-game.
"If you are going to have injuries, you'd like them to be spread a little bit across the team and not have two of your best tall forwards out."
With the absence of McStay and Daniher in the forward line, Fagan conceded that the Lions were considering bringing Eric Hipwood straight back into the senior lineup, just 10 months after the 24-year-old ruptured his ACL in a match against St Kilda.
"I'll be guided by our medical people," Fagan said.
"The medical people do such a great job now with rehabbing players from injury.
"If they're really high-quality players you don't necessarily have to bring them back through the reserves.
"We'll see how we go.
"He’s been doing match practice at training for a month now. He's been doing competitive work for the same period of time and been running big kilometres, so we’ve got a decision to make there, but he’s in great nick."