Andy Awford pinpointed the arrival of Ryan Taylor onto the Kassam Stadium turf as the catalyst for Pompey’s triumph.
The striker found himself rested and moved to the bench for Saturday’s visit to Oxford.
Instead Miles Storey partnered Craig Westcarr in attack – until the former Rotherham man’s 64th-minute introduction with the match poised at 0-0.
Westcarr grabbed the 80th-minute winner, created by Nicky Shorey with a cross from the left.
But Awford felt Taylor also played a crucial part in the victory which lifted his team into second spot.
He said: ‘When you keep giving the ball away like we did in the first half, Oxford are going to hurt you eventually because they are a good footballing team.
‘I couldn’t wait for half-time to get us in and settled down and I think in the second half we were the better side and deserved the victory in the end.
‘We wanted to keep it and get higher up the pitch, I felt a bit sorry for Ricky Holmes and Nicky Shorey in the first half, they couldn’t get over the halfway line.
‘In the system we were playing they were playing as full-backs instead of wing-backs.
‘The only way you could get them higher is to keep the ball and I think Taylor made a difference coming on with his hold-up play.
‘That helps you with his back to goal, he’s a threat, he holds the ball up for you to allow the team to get 10 yards further up the pitch which Nicky and Jed could then do.
‘We were patient when we had to be and once we got the goal it was a case of seeing it out, which we did comfortably.’
Awford maintained his preference for first-team shuffling by making two changes at the U’s.
Wallace accompanied Taylor on the bench, with Danny Hollands coming into the centre of midfield.
The youngster was introduced, though, at half-time in place of Holmes, who was walking a disciplinary tightrope.
And Awford believes the substitutions once again demonstrated his attacking intentions from the substitutes’ bench.
He added: ‘It was a tight game but I always try to win games, we don’t ever sit back.
‘Don’t get me wrong, if we went to Manchester United and it is 0-0 after 80 minutes you might look at that but I always try to win games of football, of course I do.
‘Jed needed a rest and Tayls needed a rest after what they put in the last few days and that seemed the sensible thing to do.
‘And they made an impact when they came on.’
– NEIL ALLEN