He is starting a four-week lay-off so Gannon will have to move Exodus Geohaghon back into the defence after a spell in midfield.
Record signing Ryan Bennett remains sidelined with his ankle injury while fellow centre-back Craig Morgan is also out with an ankle problem.
Posh also have problems in attack as on-loan Brighton forward Liam Dickinson remains a doubt with a hamstring injury and Aaron Mclean is out for the rest of the season after having knee surgery this week.
Gannon - who decided this week to leave the club at the end of the season - will be looking for a positive response to the 4-0 drubbing in midweek which left Posh 12 points adrift of safety.
Peterborough 6/4, Bristol City 7/4, Draw 12/5