A 1-0 Peterborough win told little of a match the Spireites performed well but couldn't take their chances in as they slipped to defeat at the Abax Stadium.
Ricardo Santos picked up to quick yellow cards on the hour mark as the Posh played the last 30 minutes a man down, but despite pressure and chances United held on to win.
Chesterfield manager Paul Cook made just one change to his starting line-up, Sam Morsy returning from suspension to captain the side, with winger Gboly Ariyibi dropping to the bench and Ollie Banks moving into an advanced attacking position.
The Spireites dominated the opening moments, with crosses from either side coming in from Ollie Banks and Drew Talbot, but dealt with by the Posh defence.
Jay O'Shea went close as the visitors continued to boss the early moments of the match. A corner was half-cleared and fell for Sam Hird to tee up the midfielder, O'Shea volleyed well but the effort whizzed over the bar.
The Spireites still in attack mode O'Shea again went close to grabbing the opener. Set free after some wonderful interplay he shot from inside the area, Ben Alnwick blocked and as the rebound returned to the Irishman his follow-up was again blocked by the Posh 'keeper.
The hosts showed the threat they carry after 20 minutes when Luke James headed wide a cross when leaning backwards to meet it, almost giving his Peterborough United side a lead that hadn't been coming.
Just after halfway through the period and the Posh took an undeserved lead. Driving into the Chesterfield box, the ball was blocked and fell to Bostwick who shot from ten yards and saw his effort deflect off a Spireites defender and roll past the wrong-footed Tommy Lee and into the net.
The ball boosted home confidence and moments later Luke james pushed in on goal and forced Lee into a fine palmed save as he tipped James's powerful effort up and over the crossbar.
Chesterfield started to find their feet again as the interval approached, but despite their efforts Peterborough took a narrow lead in at the break.
The home side nealry doubled the lead within the opening minute of the second period. James was played in after a Hird mistake and, shooting across goal, Lee saved well, and managed to pus the narrowly behind Bostwisk, sniffing around in hope of his second of the contest.
Having survived that chance, the Spireites grew back into the game, forcing Alnwick into three quickfire saves. O'Shea's 20 yard effort was too close to the Posh 'keeper. The resulting corner was then flicked goalwards by Hird, well svaed again by Alnwick.
Three minutes later and Roberts again tested Alnwick, but again the effort was straight at the United stopper who got his body in the way and retiaained his side's lead.
Chesterfield were handed a huge boost with half an hour to go when Santos received two yellow cards in two minutes and waas dismissed. The second was for a foul on Clucas, but the well-placed free kick was fired straight into the wall by O'Shea.
A scramble from a corner saw the ball break to Talbot, he shot but saw it blocked and then at the other end Conor Washington forced Lee into a good save, kneeing an effort in on goal that the Spireites number one managed to block away.
The Spireites desperate to equalise, they hit the post when Gabriel Zakuani hit his own post when attempting to play a ball back to his 'keeper and misjudging it. Unfortunately for Chesterfield the ball hit the upright and went wide and not into the net.
Ten minutes to go and Roberts forced Alnwick into a full-length save as his beautifully-controlled half-volley ws pushed behind by the Posh 'keeper - Chesterfield doing all they could to force a leveller.
Five minutes left and Morsy went close for the visitors, picking up Talbot's cleared cross and swishing a bending 18-yard effort barely a foot over the Peterborough crossbar.
Lee came up to attack a late O'Shea free kick, but the ball in was poor and the Spireites slipped to a defeat that nothing but the scoreline was indicative of.
Subs not used: Henry (gk), Vassell, McLean, Norris, Anderson, Beautyman
Chesterfield: Lee (gk), Talbot, Evatt, Hird, Darikwa (Jones 72'), Morsy (c), Ryan, Banks (Ariyibi 62'), Roberts (Dieseruvwe 84'), O'Shea, Clucas
Subs not used: Chapman (gk), Carter, Humphreys, Onovwigun
Peterborough United: Bostwick 26'
Red Card: Santos 62'