Kenny Jackett admitted his defensive concerns after Pompey again shipped in early goals on their travels.
Foundations for defeat at Doncaster were last night laid by two goals in the opening six minutes.
The Blues never recovered from that body blow as they went on to suffer a 2-1 loss, with Dion Donohue also dismissed in the second half.
It was frustratingly familiar territory for Jackett, whose team last month conceded in the 18th minute at Northampton and the 12th minute at Scunthorpe.
Pompey’s previous two league fixtures reaped clean sheets.
But a careless start at the Keepmoat Stadium ensured they would collect nothing from last night’s encounter.
It is similar to Scunthorpe and similar to Northampton, which have ultimately cost us on each occasionKenny Jackett
Jackett said: ‘I cannot fault their application, but the opening period ultimately has cost us and gave the Doncaster defence an opportunity to sit deep.
‘We weren’t second best, we weren’t outclassed, but we gave away key goals.
‘It is similar to Scunthorpe and similar to Northampton, which have ultimately cost us on each occasion.
‘For the first goal, Tommy Rowe sidestepped our right-back and one of our central midfield players too easily on the edge of the box and slotted someone in down the right-hand side and Liam Mandeville has finished the cross.
‘Then from two corners they’ve had two free headers. They’ve got one of the two and it put them in pole position at 2-0 up after six minutes.
‘The second was bit of an OG from Christian Burgess, but similarly when you give away headers like that it’s tough to defend.
‘There were actually big chances in that period and even the third one – the header – hit their guy on the line, otherwise it could have been all three.
‘There’s a long time to go and you think you can come back, but we didn’t really get the ball out to either Gareth (Evans) or Jamal (Lowe) enough to be able to open them up down that side.
‘Similarly, through the middle against three good centre-backs, we didn’t shift them around enough, we were in a too straight line and didn’t put enough balls down the side of them.
‘We also didn’t get into one-v-one situations where we were dragging them wide and taking them on.’
Jackett brought on Conor Chaplin and Jamal Lowe at the break in a bid to pep things up. The duo replaced Kyle Bennett and the recalled Evans, who was made skipper for the fixture.
And Jackett was pleased with the impact of his substitutes.
He added: ‘I felt it needed to go 4-4-2 with a second player up and around Oli Hawkins.
‘As much as anything else, just to give us a fresh momentum and something different for their defence to think about.
‘It did give us some fresh momentum to a degree, but was frustrating in the end because we didn’t get the goals back.’