Kenny Jackett believes Pompey have strengthened defensively since their last meeting with Oxford United.
The U’s visit Fratton Park today in a game that will be aired live in front of the Sky Sports cameras.
The Blues were put to the sword in the reverse fixture at the Kassam Stadium in August.
The visitors suffered a 3-0 defeat in Jackett’s third game as Pompey manager.
Despite going into half-time with the game goalless, Oxford put their foot on the gas after the restart.
Wes Thomas, Gino van Kessel and Josh Ruffels all found the net to send the Blues back down the A34 empty-handed.
Jackett admitted the loss was a ‘wake-up call’ for Pompey after spending four seasons in League Two.
And he feels his rearguard is now more adept than that meeting earlier in the campaign.
The Blues boss said: ‘Particularly with their substitutes, they did very well. They deservedly won the game in the second half.
‘As a side, it was a bit of a wake-up call for the division.
‘Portsmouth spent four years in League Two and if you get in and around the top 10 of this division then it’s a very good standard.
‘The top 10 of League One have a little bit more and I thought Oxford displayed that earlier in the season.
‘For us, new in the division, it was a wake-up call.
‘Things have changed in our defence since then and we’ve needed it right the way through the team.
‘It’s a balance between football and physical presence.’
Although Oxford clinched the three points in August, it’s the Blues who have prospered more since.
The U’s harboured play-off ambitions at the start of the campaign, but sit closer to the bottom four than the top six.
They’ve only won two of their past 12 games and sacked Pep Clotet in January.
However, Jackett believes the squad new boss Karl Robinson has inherited possesses plenty of quality.
‘Last week Oxford played a diamond formation in their win over Peterborough, with James Henry behind the front two.
‘If you’re looking right through the team, the likes of Josh Ruffels aren’t playing and they have some very good midfield players.
‘It’s an interesting squad and is very competitive.
‘They have a good squad of players and have a good model since coming out of League Two under Michael Appleton two years ago.’
Oxford finally selected their candidate to succeed Clotet on Thursday after a two-month search.
Robinson has taken the reigns at the Kassam Stadium after resigning as Charlton Athletic boss the same day.
Jackett revealed he’ll wish his counterpart the best of luck in his new post – but only after today’s game.
‘(Robinson) is a tough one to comment on because you don’t know what goes on behind closed doors at a club,’ added the Blues manager.
‘Karl is somebody I know and after the game I’ll wish him all the best at his new job.’