Gunnarsson played as a makeshift centre-back against Newcastle in midweek and was excellent before suffering a late head injury.
The Icelander needed nearly seven minutes of treatment on the pitch after cutting his head but the injury is not as serious as first feared and he may be fit to face Posh.
Gunnarsson was needed in defence after Zurab Khizanishvili suffered a knee injury. The Georgian could be back on Saturday but a late decision will be made.
Fellow centre-halves Matt Mills (suspended) and Ivar Ingimarsson (hamstring) remain sidelined leaving Alex Pearce as the only fit central defender.
Marek Matejovsky will miss out once more after he had surgery to cure his stomach problem while Noel Hunt and Chris Armstrong (both knee) remain sidelined.