The 22-year-old blue-liner did not skate with the AHL team Wednesday, still shaken following a big hit in a game Saturday against Rochester.
Marlies coach Dallas Eakins says Gardiner is suffering symptoms consistent with a concussion, though the injury hasn't been labelled as such.
"He's got a bunch of issues in his neck and some headaches, so yes it could be symptomatic to a concussion or could be symptomatic to neck pain," Eakins said at practice.
Gardiner, who missed Sunday's game against Abbotsford, will not dress Saturday in Rochester.
Eakins says the team is taking it slow with Gardiner, who would be playing for the Toronto Maple Leafs if the NHL wasn't in a lockout.
"We like to err on the side of caution and move forward as if it's a concussion," Eakins said. "He's improving, but is still symptomatic and we're very cautious with these things. We're going to take a real slow approach."
In 22 games this season, Gardiner has nine goals and eight assists. The former first-round draft pick scored twice and added an assist on Saturday before leaving the game.
Gardiner, who didn't speak to the media Wednesday, has impressed so far this season, even getting Don Cherry's attention.
"Jake Gardiner of the Toronto Marlies is not only the best defenceman but he's the best player in the American Hockey League that I have seen," Cherry tweeted earlier this week.
"He's an American, he's big, got nice hands and an edge in his play," Cherry continued.