The Australian had established himself in the side for the first time with a seven-game run before suffering the setback in early December.
McLeish admits the injury is "slightly more long term" than had been originally envisaged.
He said: "Chris has got the protective boot on again and it depends how it feels when he takes it off. That's going to be the key.
"The last time he took it off, it was still sore. It's a ligament in his ankle. It's a kind of awkward one.
"If they take the boot off and he's not got any discomfort, then it could be another month."