Midfielder Bostwick, who has been playing as a centre-half, was forced off with a hamstring problem during the second half of Posh's 1-0 win at Blackpool prior to the international break.
Should Bostwick not recover in time for Saturday, Shaun Brisley would be recalled to the starting line-up for the first time in more than two months.
New loan signing Florent Cuvelier will be involved against the Bluebirds. Cuvelier, signed from Stoke on Wednesday, could take his place in a three-man midfield alongside Jack Payne and Kane Ferdinand.
That would represent a relatively inexperienced trio though, and manager Darren Ferguson may opt for either Grant McCann or Danny Swanson.
Ferguson is expected to leave striker Tyrone Barnett on the substitutes' bench and play Lee Tomlin in support of leading scorer Dwight Gayle.
On-loan midfielders Davide Petrucci (hamstring) and Alex Pritchard (ankle) are still not fit.
Meanwhile, Malky Mackay looks set to stick with the centre-back partnership that won at Sheffield Wednesday as his side head to Peterborough.
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.