Grant Hall, Massimo Luongo, Ryan Mason and Alex Pritchard all started and all played 90 minutes at London Road.
Britt Assombalonga scored what turned out to be the winner for Posh in the ninth minute.
After that, Alex and Massimo fired just wide, Massimo was then denied by goalkeeper Robert Olejnik and with time running out, Ryan rattled the crossbar from 25 yards.
In the Championship, Bongani Khumalo made his debut for Doncaster and completed the 90 minutes in a 3-1 home defeat against Blackpool. Meanwhile, Adam Smith was an unused substitute in Derby's 1-1 draw against Blackburn.