The central defender is sidelined with an ankle injury sustained last month and is thought to be around a fortnight away from a return.
Gannon is also sweating on the fitness of defender Exodus Geohaghon and on-loan striker Reuben Reid given they are both carrying knocks, however, both are optimistic of at least making the bench.
Less promising, however, is defender Josh Simpson's chances as he is struggling with a thigh injury, although new loan signing Mark Little should come in for his first start since arriving from Wolves.
Dominic Green, who was recalled this week from his loan at Chesterfield, will vie for a place in attack after the departure of George Boyd to Nottingham Forest.
Posh, whose five wins have all come at London Road this season, have scored just five goals in their last nine league games.