Danny Cowley admits that he picked the wrong team for their Fratton defeat against Cambridge.
A fine Ronan Curtis strike ultimately proved to be too little, too late for Pompey, who slipped to a 2-1 defeat on home soil.
Goals from Joe Ironside and Liam O’Neil had given the visitors a commanding lead and they were able to see the contest out.
Cowley was disappointed with the display from his troops and knows that a big improvement is required.
The Blues boss said: “It’s incredibly disappointing and as a manager, you always take responsibility in these moments – and rightly so.
“We were well short of the performance that we wanted to produce after a good start here with two wins at Fratton Park.
“The team that we picked wasn’t the right one and we looked unfamiliar. There weren’t enough relationships out there.
“It’s easy for managers to come out and blame players – and of course they also have a responsibility – but you have two choices in these situations.
“You can either look out the window or you look in the mirror – and for me it’s always the latter of those.
"It’s incredibly disappointing and as a manager, you always take responsibility in these moments."
“We lacked the intensity that we expect from our side. We’re a high pressing team and we’ve let their centre-half travel into our final third to lead up to the first goal.
“So we didn’t follow the game-plan in that moment and then we didn’t stop the cross or deal with it when it came in. We deserved to concede.
“It took for us to concede a second goal to show any aggression or purpose in our play, which is bitterly disappointing.
“I think that’s only the second time in my managerial career that I’ve lost three league games on the spin.
“It was well short of what our supporters deserve. There’s rightly a lot of expectation and the shirt is heavy, but we have to be much braver.”
Pompey were dealt a blow before the game, with skipper Clark Robertson and fellow defender Connor Ogilvie both ruled out.
Cowley said: “We haven’t had a good week, losing Connor and Clark – our two left-footed centre-halves.
“It disrupted us and there’s no hiding away from that, but these things happen and you have to overcome it.
“We don’t think Connor will be too long, but Clark has to see a specialist and we’ll know more off the back of that.”