The Lions are sweating on the results of Gardiner's scans after he pulled up sore and hobbled off the field late in the 19.9 (123) to 14.17 (101) win over Geelong on Thursday night.
The 30-year-old had been a key contributor, limiting out-of-sorts Cats star Jeremy Cameron to just two disposals before leaving the arena midway through the final quarter.
"Darcy might have a decent hamstring injury," Fagan told reporters post-match.
"Don't know the extent of it yet, obviously, which is a pity for him.
"He gets injured a lot and played a pretty decent game tonight on Cameron until that moment. Fingers crossed it's not a serious one."
Gardiner has managed just 38 games since the start of 2023 amid a series of injury issues, including significant shoulder, knee and ankle setbacks.
He missed six weeks with a knee injury earlier this season.
The outlook is better for teammate Zac Bailey, who starred with 20 disposals and three goals against Geelong before sitting out the dying stages with hamstring tightness.
"We just didn't take any risk with him once we knew we had the game won, so I wouldn't expect him to miss (any games)," Fagan said.
Lachie Neale passed a concussion test in the second quarter and was another important contributor with 28 touches and six clearances.
Neale was sent off the field by umpires, who stopped play after being notified by the AFL Review Centre that the star midfielder required attention.
"He got checked out, he was passed to be OK and continued on for the rest of the game," Fagan said.
"I thought that was all done pretty well."
Brisbane's fourth-straight win meant they leapfrogged Geelong into fourth spot on the ladder with a 10-6 record ahead of a home clash with bottom-side Essendon next week.
Fagan expects Hugh McCluggage (hamstring) and Dayne Zorko (quad) to return from injuries against the Bombers, while Oscar Allen (foot) is likely a further week away.
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