Gallas, who has a history of calf issues, suffered his latest setback during the opening half of Glory's 2-1 loss to Central Coast Mariners last Saturday night.
Fellow defender Scott Jamieson will miss the rest of the season after tearing his Achilles tendon in the same match, while skipper Jacob Burns was handed a two-match ban for stepping on Mariners midfielder John Hutchinson.
Glory owner Tony Sage has threatened to take legal action unless Football Federation Australia overturns Burns's ban.
Gallas, who was lured to Glory on a one-year deal believed to be worth $700,000, said he was disappointed to succumb to injury just 30 minutes into his run-on A-League debut.
"I've got a little problem with my calf. I did an MRI, so I think I would be injured for up to three weeks," Gallas told Thursday FC.
"The speed of the game goes very fast. I didn't expect that.
"I have seen some quality players.
"In my team we have some young players, and I'm sure they can go very far.
"The league will be very tough because nobody knows who's going to be champions. We still have a chance to be champions."
Gallas looked every bit of a marquee player against the Mariners, and almost scored a goal with a stunning bicycle kick manoeuvre.
"When I was young I was a striker. Maybe it came from that skill I had when I was young," the 36-year-old said with a laugh.
Steve Pantelidis will replace Gallas in the heart of defence, while either Jack Clisby or Riley Woodcock are set to fill the void left by Jamieson.
Seventh-placed Glory takes on Brisbane Roar at Suncorp Stadium this Saturday before returning home for a clash with Wellington Phoenix.