Duncan hurt his right knee in a marking contest during the opening stages of Friday night's thrilling five-point win over the Western Bulldogs.

The 30-year-old failed a fitness test on the boundary line and was replaced by medical substitute Shaun Higgins.

Geelong have not put a timeline on Duncan's return but the PCL injury is expected to keep him out at least until the start of the finals.

"We were expecting that the injury was to Mitch's PCL, and the scan confirmed that," Cats football boss Simon Lloyd said.

"There was no other damage to Mitch's knee. The rehabilitation timelines will be further discussed with our medical and fitness team in consultation with specialists.

"We will explore all available options that will allow Mitch to return as quickly and safely as possible."

Duncan missed the first two games of the season but has been one of the Cats' most crucial contributors since.

He is one of only three survivors from Geelong's last premiership in 2011 with fellow veterans Joel Selwood and Tom Hawkins.

After pipping the Bulldogs thanks to a Gary Rohan goal after the siren, the Cats face another top-four contender in Brisbane at the Gabba on Thursday night.

But Geelong will have Selwood available for the trip north after the Cats captain escaped with fines for two separate incidents against the Bulldogs.

The Cats and Lions are equal second premiership favourites, narrowly below ladder-leaders Melbourne.

Australian Associated Press