Michael Appleton blasted the Pompey defending which he believes set Millwall on their way to a crucial victory.
The Blues could now be relegated at Doncaster on Saturday following last night’s hugely-disappointing 1-0 defeat.
The Lions’ on-loan Spurs striker Harry Kane settled the issue in the 37th minute with a smart finish.
Yet for Appleton, his side’s inability to deal with the danger earlier in the move proved so costly.
Kane’s decisive goal came at a time when the hosts were in charge of the match and were looking comfortable at the back.
Pompey never recovered and rarely threatened for the remainder of the match, where they looked lacklustre in the second half.
Appleton said: ‘We started the game really, really well for the first 15 minutes or so.
‘I can’t remember them looking really threatening at all. There were quite a few good opportunities when we got in behind them, but the first goal in football is so important.
‘We have allowed a centre half to lump the ball up the field, not deal with the first one, not deal with the second one and then gave a really poor goal away.
‘As soon as you do that, you are on the back foot.
‘It was a poor, poor goal to give away. It is not even a danger if we deal with it properly.
‘I have said it numerous times – it doesn’t matter if you are in the Champions League or the Conference, it is so important to get that first goal, especially when you are on the front foot, especially when you are in the ascendancy.
‘To concede the goal we did it was always going to be hard work because you have got to give credit to Millwall.
‘They came here with two banks of four, sat them behind the ball, had two centre forwards who work ever so hard and it suits them playing away from home.
‘With their style of play, they won the second balls, are very competitive, play behind the ball and it suits them.
‘Once they get ahead it suits them even more.’
Pompey’s display in the second half was even more disappointing as they failed to respond to Kane’s goal.
It wasn’t until the final five minutes, when Maik Taylor pushed Jason Pearce’s header against a bar and then spectacularly saved from Kelvin Etuhu, that the hosts threatened once again.
The result means relegation to League One now appears to be a certainty for Pompey.
Appleton added: ‘The amount of time we got into good areas and over-hit passes, it was a difficult night.
‘It got to that point towards the end where a few of the players were really suffering. They were looking at each other waiting for someone to do a bit of inspiration.
‘It didn’t happen – but it nearly did.
‘Kelvin Etuhu had a fantastic strike and Maik Taylor – the old fox – was there and pulled off a fantastic save because it was going into the far corner.’