Lynch sustained an accidental head knock from Darcy Fort in the dying stages of Sunday's thrilling five-point win over Brisbane in Launceston and spent the last few minutes on the bench.
Hawks coach Sam Mitchell said post-match he thought Lynch would be given the all-clear to play in round 11.
But the 200cm ruckman failed a test on Monday and has been placed in concussion protocols.
Lynch sat out multiple weeks after suffering concussion from a head knock during his club debut for Hawthorn in round one.
He has now been added to an injury list that already includes captain Ben McEvoy (neck) and novice ruckman Ned Reeves (shoulder).
The situation leaves Mitchell with a problem to solve ahead of the match against Gold Coast in Darwin on Saturday night.
Tall midfielder Conor Nash and key forward Jacob Koschitzke have both pinch-hit in the ruck this season and could be called to take on in-form Suns big man Jarrod Witts.
Hawthorn midfielder Will Day is also in doubt this week after being substituted out of the Brisbane game with an ankle injury.
It is the same ankle Day broke last year but the latest injury is a jarring issue that occurred when he slid into a fence while trying to save a goal.
Hawthorn's win over the Lions snapped a four-match losing streak and rekindled their finals hopes.
The Hawks' 4-6 record has them sitting two wins and percentage outside the top-eight.
19°C