Sydney also lost Justin McInerney to a hamstring injury and several teammates were sore after Sunday's pulsating two-point win over St Kilda at the SCG.
McCartin bravely went back with the flight of the ball in the third term and Saints small forward Liam Ryan cannoned into his back.
Swans coach Dean Cox revealed there was no assessment for McCartin before he was ruled out of the game.
"If you are out for a period, you fall straight into the concussion protocols," Cox said.
McCartin has missed several games through concussion, most recently last year. His brother Paddy was forced to retire prematurely three years ago because of his concussion history.
"He's obviously disappointed, but he feels fine," Cox said.
"The doctors ruling him out was obviously the right thing to do, and now it's about making sure we look after his health.
"By all reports he's in great spirits, so that's the first thing.
"But we understand he's concussed and we'll manage that accordingly."
McInerney will go for scans and the Swans are bracing for bad news, given he could not return to the ground.
"Obviously, not finishing the game doesn't look great," Cox said.
With McCartin on the bench for much of the second half, and fellow backmen Lewis Melican and Dane Rampe also out of the game, Cox was rapt that younger defenders such as Harry Kyle were able to step up and hold the Saints at bay.
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