The midfielder is poised to have a fitness test on an ankle injury which he picked up in training.
Should Williams be ruled out, Jamie Day is pushing for a recall after playing an hour in the club's reserve side on Wednesday.
Manager Darren Ferguson is definitely without the services of midfielder Paul Coutts, who is out for two to three weeks with a foot problem.
Centre-half Craig Morgan is expected to start despite being on international duty for Wales in midweek.
Highly-rated George Boyd has been told he will start by Ferguson and the striker will equal Eric Steele's record of 124 consecutive Football League appearances for Posh.
