McCready injured his hamstring in the closing stages of the 2-1 defeat at Aldershot in midweek and is set for a fortnight on the sidelines.
Fellow centre-half Dean Beckwith remains out with the same injury but the Cobblers are hopeful Danny Swailes will be granted permission to make his debut after signing on loan from MK Dons.
On-loan Middlesbrough full-back John Johnson is a doubt to feature thanks to yet another hamstring injury while forward Adebayo Akinfenwa is also a doubt after limping off against the Shots with a sore knee.
Courtney Herbert and Abdul Osman remain out with back injuries, while winger Liam Davis has suffered a setback in his recovery from a broken foot and will be out for another three weeks.
Peter Crouch, Kevin Phillips and Jamie Redknapp were on the scoresheet the last time the Saints travelled to Sixfields for an FA Cup tie in 2005.