It was initially thought Paynter's injury, collected on the tour to Slovakia, was a minor hindrance and he was expected to return at Hartlepool tomorrow.
However, after a reassessment, the former Swindon man now looks set to miss the early weeks of the Championship season.
With Jermaine Beckford gone to Everton and Davide Somma and Max Gradel facing suspensions from the season's start, manager Simon Grayson is very short of attacking artillery.