Deputy vice-captain Michael Rischitelli also had an MRI on his elbow that revealed ligament damage.
He will see a specialist later in the week to determine a further course of action.
Both players suffered their injuries in Saturday night's loss against Essendon.
Lynch injured his knee in a contest with Jake Melksham during the second quarter and Gold Coast's general manager of football operations Marcus Ashcroft said it was too early to confirm a return date.
"The results of Tom's MRI have confirmed a PCL strain," Ashcroft said.
"The exact timeframe that Tom will be sidelined for is unknown and will become apparent as he works his way through the rehabilitation program."
Rischitelli's timeframe on the sidelines is also unknown.
The injuries come as the Suns just begin to get some players back.
Defenders Nathan Bock (leg) and Seb Tape (knee) have played a few weeks in reserves from their long-term injuries.
Harley Bennell and Karmichael Hunt are both expected to be available for senior selection after the Suns' bye after missing through hamstring problems in recent weeks.
Michael Whiting covers Gold Coast news for AFL.com.au. Follow him on Twitter: @AFL_mikewhiting
