The 36-year old French defender, formerly of Chelsea, Arsenal and Tottenham fame, sustained the problem in Saturday's 2-1 loss to Central Coast Mariners which was his first start for the A-League club.
Gallas told SBS' 'Thursday FC': "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.
"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."
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