On-loan winger Pritchard, 18, suffered an ankle injury during Peterborough's last match against Leicester on February 9 and the player is back at his parent club Tottenham receiving treatment. He has been ruled out for around six weeks.
However, Kgosi Ntlhe, Craig Alcock and Jaanai Gordon-Hutton will come back into the reckoning.
South African Ntlhe has been out since the start of November with an ankle injury.
Fellow defender Alcock and forward Gordon-Hutton both missed the 2-1 home win over Leicester nine days ago with knocks.
Bottom-of-the-table Posh head to London looking to extend their three-game unbeaten run in the npower Championship.