Kenny Jackett rued an ineffective opening 45 minutes as Pompey suffered a first defeat in 14 matches.
A club-record pursuit of nine-successive wins ended with a 2-1 defeat against Charlton at Fratton Park last night.
The first loss in more than two months was underpinned by an awful first-half display in which Lee Bowyer’s side established a 2-0 lead.
Karlan Grant and Nicky Ajose capitalised on arguably the Blues’ worst opening half performance during a season which has seen them top League One.
Yet it was a completely different Pompey after the break, albeit frustratingly only having substitute Andre Green’s late consolation to show for a much-improved display.
And while Jackett was encouraged by his side’s heart and drive in their attempts to fight back, he regretted that first-half contribution.
He said: ‘Charlton are a good side, they have some Championship quality players, they were a play-off team last year and have aspirations to do that again this year, obviously.
‘But we didn't get to grips with them in that first half and didn't give them enough problems. I cannot really think of a good move or situation (in that period) where we created enough, and that gave Charlton the initiative.
‘In the second period we responded well and I did feel by the end were unlucky not to get anything out of the game.
‘We had to change one or two things and play with a better tempo. I did think that, even at 2-0 down, the game was still there for us.
‘I didn't think it was gone and there was a big comeback and a big performance in us.
‘We tweaked it a little bit. Then Brett came on for Oli, it wasn’t not quite Oli’s day, he was quite quiet, and we had Brett with his experience and hopefully his hunger. I felt he responded well when he came on.
‘We couldn’t quite get that comeback, despite putting a lot of pressure on them and getting the crowd going as well, which was great, they were behind us and bouncing.’
Pompey remain six points clear at League One’s summit having lost their game in hand.
Now they head into top-of-the-table clashes at Barnsley on Saturday, followed by the visit of Sunderland (December 22).
And Jackett feels they can take their second-half showing into those crucial fixtures.
He added: ‘In the second half we came back with a performance we can base things on and be positive about.
‘Ultimately, though, we just couldn’t get back into the game.
‘We have to take the second-half performance and learn from the first half, which was a slight role reversal of many of our games this season where we had the initiative early and then defended in the second half.
‘It was hard for us in that second period, although I did think we made a fist of it.’