The centre-half, who has only recently returned from an eight-month absence with knee ligament damage, was forced off during the midweek win at Port Vale after suffering another injury to the same knee. It is not yet known how long Downes will be out this time.
Carlisle striker Gary Madine will go straight into the squad after arriving on a month's loan. He has been brought in as cover for leading scorer Jack Lester, who is set to be out for up to six weeks after undergoing knee surgery.
Peterborough winger Dominic Green has extended his loan by a further month - it was set to expire on Friday - and remains available.
Phil Picken will be out for up to three weeks after the defender suffered a knee ligament strain in training.
Chesterfield 11/8, Bury 15/8, Draw 12/5
Source: Team Talk
Source: Team Talk