The Australian centre-back has been sidelined for past 12 months with cruciate ligament damage.
Downes' last first-team appearance for the League Two club was exactly a year ago when he sustained the injury during a 2-1 victory at Port Vale.
"He's not going to figure this season," Chesterfield boss John Sheridan told BBC Radio Sheffield.
"He's had to have a trim on his knee, his cruciate is fine, his knee is very strong. It's just a niggle but it's something that needed doing."
Source: PA
