Ben Turner was partnered by Norwich loanee Leon Barnett in the 2-0 victory at Hillsborough and with captain Mark Hudson sidelined again with his ankle injury, the pair are likely to get the nod once more.
Meanwhile, Mackay has confirmed that midfielder Jordon Mutch is in full training and ready to go having been troubled by a niggling foot injury.
And the Scot will monitor the fitness of Welsh striker Craig Bellamy ahead of the trip.
The former Liverpool man, 33, played 180 minutes for his country as they beat Scotland and lost to Croatia in their World Cup qualifiers, and Mackay may decide against letting the player start for the third time in eight days.
Cardiff are seven points clear at the top of the npower Championship but Peterborough have been something of a bogey team for the Bluebirds in recent seasons. Posh have lost just two of the last seven meetings between the sides.