Giles Coke has suffered an injury setback on the eve of his much-anticipated debut for the Cobblers.
The midfielder has been out for 11 months with a knee problem, but has made several reserve-team appearances and been in full-contact training for some time now.
Manager Stuart Gray was planning to play his summer signing in the FA Cup second-round game against Walsall, but now won't be able to, with Coke injuring his groin in his latest run-out for the seconds.
Gray will also be without loanee Dean Bowditch, who has not been given permission to play by parent club Ipswich, but Guy Branston can appear after getting the green light from Peterborough.
Ian Henderson may also be involved after coming through Tuesday's reserve-team game with no adverse effects.