Gary Roberts first half goal means that Spireites win the game no-one wanted and will now travel to Scunthorpe on Tuesday in the FA Cup third round.
Man of the moment Eoin Doyle remained in the starting eleven as Paul Cook made just one change with skipper Ian Evatt out after picking up a calf injury training on AstroTurf on Thursday. Sam Morsy returned after suspension and took the armband, with his return meaning Sam Hird switched from the centre of midfield to the heart of the defence.
Chesterfield started the high pressure, high pressing game that earned them a win in the original fixture, only for the eligibility of loaned Georg Margreitter to render that victory irrelevant.
That said, the hosts got early efforts in from Danny Green and Benik Afobe before Doyle, with Derby County’s Chris Evans watching, headed just wide.
Former Spireites Mark Randall hit high before Spireites began to take charge. Armand Gnanduillet headed home only to be denied by an offside flag whilst Doyle blazed over from a great one on one opening.
On 43, Doyle found himself in a similar position but this time left winger Gary Roberts was in support and in space and the under the microscope, the Irishman rolled the ball to his colleague who had the time to control and slot home from eight yards to give the visitors a deserved lead.
Gnanduillet was the focal point after the break, shooting and heading within a whisker and seeing David Martin make several saves.
The Dons started to build up a head of steam as the game went on but the truth is they only managed two shots on target all evening.
Throughout the entire game the visitors were more determined, tougher and more resilient, all properties that manage Cook will have been delighted to have seen.
MK Dons: Martin, Lewington, McFadzean, Green (Carruthers 59), Potter, Bowditch, Spence, Randall (Reeves Ht), Powell (Griggs 59), Baldock, Afobe. Subs: McLaughlin, Hodson, Flanagan, Alli.
Chesterfield: Lee, Darikwa, Hird, Raglan, Clucas, Roberts, Morsy, Humphreys, Boco, Gnanduillet, Doyle. Subs: Wright, Jones, Gardner, O’Shea, Talbot, Ariyibi.
Ref: Steven Martin (Staffs)