Pompey produced a performance full of heart, desire and effort in their League One contest at high-flying Peterborough.
But – in the first game since Kenny Jackett’s departure – it ultimately proved not enough to put a halt to their run of losing form.
An unfortunate Rasmus Nicolaisen own-goal in the first half was enough to secure all three points for Posh.
And although the Blues caused plenty of problems after the break, they were not quite able to force an equaliser.
Caretaker boss Joe Gallen made five changes from the side that were beaten on penalties by Salford in Saturday’s Papa John’s Trophy final.
Andy Cannon returned following a hamstring problem, while Ben Close was handed a first league start in exactly four months.
Nicolaisen, Lee Brown and Ronan Curtis also came into the line-up, as the visitors switched to a 3-1-4-2 system.
Jordy Hiwula, Harvey White, George Byers and Charlie Daniels all dropped to the bench, but an ankle injury forced John Marquis to miss out completely.
There were a couple of familiar faces in the Peterborough team, with former Fratton full-backs Nathan Thompson and Dan Butler both named in their defence.
Pompey looked organised and kept their discipline right from the off, preventing the hosts from being able to test Craig MacGillivray.
Passing football was proving difficult for both sides on a poor pitch that immediately began to cut up even more.
Ryan Williams was looking the most dangerous player for the visitors and had a couple of dangerous runs that ultimately came to nothing.
The Australian forward then sent a dangerous delivery into the box that Curtis could not quite bring under control.
And Thompson then made a key challenge to stop Williams getting a shot away after a long Jack Whatmough throw had been flicked on.
But Posh took the lead in fortunate fashion on 32 minutes, when Harrison Burrows swung in a dangerous corner.
MacGillivray tried to palm the ball away to stop it crossing the line, only for it to hit team-mate Nicolaisen and ricochet into the net.
Pompey’s best chance of the half soon followed, as a fiercely-struck Curtis free-kick was well held by Christy Pym.
There were a couple more half-chances for the visitors as they finished the half with a flourish, but they could not find a leveller.
Half Time: Peterborough 1 Pompey 0
The Blues caused some problems when the action restarted and a deep Brown free-kick had to be nodded behind.
Curtis sent the resulting corner in the box, but James Bolton was unable to keep his header down at the back stick.
And a second set-piece soon followed from the Republic of Ireland international, with Posh failing to clear and Nicolaisen seeing his eventual effort deflected wide.
Whatmough then had a decent chance to level when he rose highest to meet Brown’s free-kick, only to divert the ball well wide.
Gallen made his first switch just past the hour mark, with Marcus Harness providing a fresh attacking option in place of Cannon.
Pompey were looking the more threatening side, but MacGillivray was still called into action keep out an attempt from substitute Mo Eisa.
And the keeper then had to dive to his right to push away an instinctive long-range strike from Louis Reed.
A double switch followed for the visitors, with Sean Raggett and Williams making way for White and Hiwula.
Set-pieces were looking the Blues’ most likely source of an equaliser and another Brown free-kick caused a few problems before Bolton nodded over.
And Tom Naylor did the same when Curtis sent one in from the other side after Mark Beevers had been booked for fouling Harness.
Hiwula then hit a really crisp shot from open play, but the ball flew straight at a grateful Pym, who comfortably held.
And White was the next player who could not keep his header down, this time when Harness swung in a fine cross.
It had been a performance full of endeavour from the Blues, but their opponents did just enough to hold on for victory.
Peterborough (3-4-1-2): Pym; Kent, Thompson, Beevers (c); Ward, Brown, Reed (Mason 83), Butler; Burrows (Blake-Tracy 89); Clarke-Harris, Dembele (Eisa 63)
Goals: Nicolaisen 32 (og)
Subs not used: Gyollai, Barker, Kanu, Jones
Pompey (3-1-4-2): MacGillivray; Whatmough, Raggett (White 73), Nicolaisen; Naylor; Bolton, Close, Cannon (Harness 61), Brown; Curtis, Williams (Hiwula 73)
Booked: Whatmough, White
Subs not used: Ward, Mnoga, Daniels, Byers
Referee: James Oldham