The 26-year-old, who followed Dave Penney to Boundary Park from Darlington, is recovering from a stomach strain.
Penney said: "He is about a week behind the rest of the lads fitness-wise and has a little catching up to do.
"Rob has been working with the physio and the last thing we want to do is push him too hard."
Purdie has already been ruled out of Thursday night's pre-season friendly at Halifax and could also miss Saturday's trip to Bury.
Penney added: "Realistically, we are looking at next week."