The defensive solidity so impressively evident at Kingsmeadow crumbled alarmingly as Pompey handed Northampton victory.
Granted, in appalling weather conditions consisting of driving rain and swirling winds, last night’s fixture had the potential for yielding costly mistakes.
So it proved, with Kenny Jackett’s men succumbing to some shoddy moments at the back in contrast to their Wimbledon strength.
The Cobblers found themselves in a 2-0 lead after 32 minutes and, despite a spirited second-half showing from the Blues, they never surrendered their advantage.
Pompey even had opportunities to level once Matty Kennedy had pulled one back in the 49th minute to make the scoreline 2-1.
However, Jamal Lowe missed both when he truth he should have netted to restore parity.
As it was, Chris Long was allowed to finish unmarked at the far post 20 minutes from time to seal a 3-1 victory.
Northampton keeper Matt Ingram produced several excellent saves to keep the Blues at bay late on and other attempts fizzed over the bar but it wasn’t their evening.
Certainly, there will be a lengthy inquest into the manner of the three goals conceded – with Luke McGee under scrutiny over the opening two.
For everything to admire about defensive organisation at Wimbledon, crucially it was lacking at Sixfields – and proved the undoing of Jackett’s troops.
The Blues boss had to make an enforced change to the side which won 2-0 at Kingsmeadow on Saturday.
With Oliver Hawkins recovering following six stitches in a head wound, Conor Chaplin was the natural replacement in attack.
The youngster scored within five minutes of entering that Kingsmeadow encounter from the substitutes’ bench and represented the sole alteration to that starting line-up.
That was his second goal of the season – both arriving from the bench – as he seeks to become a regular starter under summer managerial appointment Jackett.
Elsewhere, Curtis Main came on to the bench as Pompey’s striking back-up, having dropped out of the squad in recent weeks.
However, once again there was no room for Gareth Evans, Danny Rose and Brandon Haunstrup in the match-day 18.
The game sprang into life on 10 minutes when Stuart O’Keefe produced a wonderful piece of skill and power to burst through a ruck of home players and charge upfield.
By the time he broke into the Cobblers penalty area he was crowded out and the ball was lost as he struggled to find decent support.
Moments later, Thompson’s clumsy challenge on George Smith earned him the first yellow card of the game.
On 13 minutes, Kennedy provided the Blues’ first attempt on goal – a right-footed shot from outside the box which curled just wide of the far post.
At the other end, there was a scare for Pompey when Matt Grimes’ free-kick into the box from the right was completely missed by McGee and Matt Clarke.
Fortunately the Blues escaped, the ball running out for a goal-kick, although the home fans were baffled how the visitors were not punished in that moment of action.
The deadlock was broken on 18 minutes through Long’s composed finish.
Matt Crooks started it off by cutting inside before feeding Shaun McWilliams.
He slid a superb ball through to Long, who made no mistake with McGee to beat.
Northampton were forced into a change on 29 minutes when McWilliams sustained an injury in a challenge with Adam May.
He was replaced by Yaser Kasim.
The Blues fell further behind on 32 minutes when another Grimes free-kick from the right caused problems and Clarke put through his own net via his chest.
Pompey were nearly handed a lifeline in the 38th minute through a misjudgment by Ash Taylor.
Burgess’ ball out to the right released Lowe and when he fired in a cross, Taylor planted a diving header against his own post before the hosts scrambled it clear.
With a flurry of injuries, seven minutes were indicated as time added on to a half the Blues would want to swiftly forget.
There was an O’Keefe effort pushed out for a corner by keeper Ingram but it remained 2-0 at the interval.
Within four minutes of the restart, the visitors had reduced the deficit.
O’Keefe scampered into the box and clipped in a cross from the right, with Kennedy firing in a low first-time right-foot shot into the corner. Moments later and the match should have been all-square when Damien McCrory floated in a cross to the back post from the right.
Lowe was all alone but, from four yards out, could only plant his header the wrong side of the post to spurn the chance.
On 57 minutes, Lowe had an even better opportunity when the ball fell to him eight yards out with an free net ahead.
This time he could only shoot it right-footed wide.
The Cobblers then regained their two-goal advantage on 70 minutes when a cross came in from the right and Long had time at the far post to place his finish.
On 83 minutes, McCrory’s free-kick from the right found Burgess at the far post.
However, the Pompey defender’s header was brilliantly stopped by Ingram.
The keeper then brilliantly flung himself to his right to tip away O’Keefe’s drive from outside the box which had the Blues man staring in disbelief.
There was to be no coming back for the visitors and the fixture finished in a disappointing 3-1 scoreline.