The star ruck suffered a head knock after being crunched in a marking contest in the final quarter of a thrilling two-point win over Brisbane at the Gabba on Saturday.
Grundy had passed a head injury assessment and was on his feet after the match.
But Sydney coach Dean Cox confirmed Grundy reported delayed symptoms the day after their Gabba clash.
"He obviously passed his HIA but then wasn't well yesterday," Cox told SEN Breakfast on Monday.
"He'll be into concussion protocols, which will mean he won't be available this weekend."
It is the first time Grundy will miss a game for the Swans, having featured in 47 matches since moving from Melbourne at the end of 2023.
Grundy could return to play in Sydney's final-round clash against West Coast should he progress through concussion protocols.
"He might be back for that," Cox said.
"I think the thing we need to do with Brodie is to just assess day-to-day, which is what we have done with Hayden McLean.
"He (McLean) came back after the 12-day protocol, and Tom McCartin was a week earlier.
"We'll just assess it as it plays."
Swans defender Justin McInerney is in doubt to feature again this season as he awaits scans on a likely posterior cruciate ligament injury.
McInerney injured his knee in the final quarter against the Lions.
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