Andy Awford criticised Pompey’s defending after Ricky Holmes returned to haunt his former employers.
In the 25th minute, the Northampton winger picked up Chris Hackett’s throw from the right and cut across the penalty area before curling a magnificent left-foot cross into the far bottom corner.
It was a sweet moment for Holmes, who was shown the exit by Awford in January, and he celebrated accordingly, with no interest in toning down his reaction in front of the travelling supporters.
That moment proved decisive, with Chris Wilder’s men running out 1-0 winners.
It ended Pompey’s seven-match unbeaten run at the venue where their Football League survival was secured last season.
And, of course, there was also the factor of Holmes, a player Awford fell out with before sending him packing to Northampton in the first place.
He said: ‘We switched off from a throw-in. We had the ball attacking up our left-hand side, they clear it, it goes out for a throw-in and we haven’t reacted quickly enough.
‘It wasn’t even a quickly-taken throw-in either but we haven’t got organised and Ricky has finished it well, you have to say that.
‘But if you give any player that time and space from a throw-in you are going to come unstuck at some point and unfortunately that is what happened.
‘He had too much time and space, anybody allowed that would probably hurt you but he did finish it well.
He had too much time and space, anybody allowed that would probably hurt you but he did finish it well.Andy Awford
‘With former players coming up against you sometimes it happens, sometimes it doesn’t.
‘I think Cambridge picking Ryan Bird the other week probably had that in their plan and it didn’t happen. Last night obviously it did.
‘I am just disappointed we haven’t managed to score.’
Meanwhile, Ben Chorley will miss Saturday’s trip to Accrington Stanley following his stoppage-time dismissal.
The Blues skipper had already been booked for a foul on Ivan Toney, then collected a second in an aerial challenge with Brendon Maloney.