Colchester boss John McGreal admitted his side were too naive during their 4-0 thrashing at the hands of Pompey.
The Blues dominated proceedings at the Weston Homes Community Stadium and could easily have had more to show for their efforts in a one-sided affair.
Eoin Doyle opened the scoring for the visitors midway through the first half.
Pompey then piled on the misery for the hosts with a superb second-half showing that produced goals for Kyle Bennett, Danny Rose and Michael Doyle.
With 13 first-team players currently overcrowding the Colchester treatment room, McGreal knew he had a job on his hands to match the Blues’ prowess.
But he still felt his side made it too easy for Paul Cook’s troops.
Speaking to the Colchester Gazette, McGreal said: ‘I thought we started it quite brightly but they showed a lot of bravery at the right times and we showed a lot of naivety and took a backward step really.
‘Their bravery showed more and that’s what dictated the game.
‘I was quite content at half-time despite the scoreline but the second goal has killed us.
‘We switched things around and made a substitution but the second goal was a bad one to give away and you’re playing catch up then.
‘At 1-0, you know you’ve got an opportunity, but at 2-0 you’re chasing it and they’ve picked us off.
‘It was a tough day for a young team, even against a team that’s been flying away from home.
‘Portsmouth are a good team – it would have been nice had we had our strong team against their team but it’s not been that way.
‘You can’t take anything away from them – I thought they were excellent.’
While victory consolidated the Blues’ place in League Two’s top three, defeat dealt a blow to the U’s own promotion hopes.
They now sit 10th in the table – three points adrift of the play-off places.