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 and had still not recovered in time for the visit of Cardiff on Saturday.
Bostwick was rated at 50/50 ahead of the 2-1 victory over the npower Championship leaders.
Florent Cuvelier was included in the squad against Cardiff but the on-loan Stoke midfielder did not make it off the bench and will hope to make his debut at the Riverside.
On-loan midfielders Davide Petrucci and Alex Pritchard (ankle) are still not fit, with the former having returned to Manchester United after failing to recover from a hamstring injury.
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