Number eight Vunipola will be sidelined for at least three months after injuring a knee against Argentina at the weekend and now faces a race against time to be fit for the Six Nations.
Of more immediate concern for the Red Rose is the visit of the Aussies and trying to keep coach Eddie Jones' perfect record intact.
Wasps forward Hughes missed the 27-14 win over the Pumas due to a foot problem but is now back among the squad and, according to Gustard, ready to play, with Jack Clifford, Josh Beaumont and Teimana Harrison also possibles to fill in.
"He's had a weekend off, he's feeling fresh, and he'll crack on with training today," he told BBC Radio 5 live. "We've got options. We will look around at what the best fit is and we will try some different combinations [in training].
"When somebody gets injured, it is an opportunity for somebody else to step in. Other players need to step up to the mark and take their chance."