James Collins gave champions Northampton a 2-1 win over Pompey today.
The substitute struck with nine minutes left to ensure the Blues will have to go to Plymouth for the second leg of the play-offs next Sunday.
And they will have to do that without Kieron Freeman who was sent off for a reckless second-half lunge on John-Joe O’Toole.
Freeman’s season is over after the dismissal, giving Paul Cook no cover at right-back for the remaining games of the season.
Kal Naismith cancelled out Jack Whatmough’s own goal with a peach of 35-yard strike after 48 minutes.
But Collins then settled the encounter to ensure Pompey finished in sixth place.
Pompey had the best early chance when Adam McGurk headed Brandon Haunstrup’s cross wide.
But Whatmough’s misfortune saw him deflect Adams’ cross past Brian Murphy 60 seconds later.
Kal Naismith headed wide from Gary Roberts’ free-kick after 26 minutes.
Roberts had a shot blocked four minutes later before John Marquis blazed a shot over from 12 yards at the other end.
Naismith then missed a gilt-edged chance when he somehow failed to force Ben Davies’ cross home from a yard out after 33 minutes.
Roberts’ attempted lob failed to extend Adam Smith, before the last chance of an open half saw Nicky Adams volley over when well placed.
Pompey’s task of levelling was made harder two minutes after the restart after Freeman’s late lunge on O’Toole.
But the response was instant with Naismith’s cracking effort from 35 yards beating Smith.
Marquis extended Murphy with a drive from inside the area after 65 minutes.
Conor Wilkinson was handed his Pompey debut in place of Whatmough with 21 minutes left.
Naismith then had to make an important clearance off the line after a flowing Northampton move.
But the second goal arrived with nine minutes left, as sub Collins deftly tucked the ball home at the near post from David Buchanan’s cross.
That proved crucial as the champions held on to collect the three points.