Goals from Wes Thomas, Gino van Kessel and Josh Ruffels ensured Pompey endured a disappointing afternoon at Oxford.
And Kenny Jackett felt that they did not make the hosts work hard enough for their Kassam Stadium victory.
Brett Pitman had a goal controversially disallowed just before the break when the game was still goalless.
But Jackett does not want to use that as an excuse for why his side suffered their first league defeat of the season.
The Blues boss said: “We produced a pretty decent first half performance and were unlucky not go ahead.
“It looked a good goal from Brett, but you always get decisions like that and we can’t feel sorry for ourselves.
“The harder we tried going forward in the second period, the more open we looked at the back.
“Their substitutes were more incisive than ours and had a bigger effect on the game, particularly on the break.
"The harder we tried going forward in the second period, the more open we looked at the back."
“Oxford were happy to sit deep and we didn’t take our 1v1 situations to actually get past people.
“We consistently made the wrong decisions and that resulted in them looking more dangerous on the counter than we did in possession.
“We didn’t do enough as a team and we made it too easy for our opponents. They’re a good side, but we want to do well ourselves and perform better than that.”
There was more bad news for Pompey, with the news that injured left-back Tareiq Holmes-Dennis faces a lengthy spell on the sidelines.
Jackett said: “It’s not good. He needs a knee operation on Monday because the cartilage is quite badly damaged and I would think he’ll miss the remainder of the season.
“We wish him all the best and he will get back because he’s a determined man. He’s young enough to bounce back.”