Guy Whittingham rued the poor defending which condemned Pompey to a 4-1 defeat against Oxford.
The Blues got off to the worst possible start to their League Two campaign, their first as the UK’s biggest community-owned club, in front of 18,181 fans at Fraton Park.
Pompey started impressively and had the lead through Patrick Ageymang.
But poor defending cost them two goals before Johnny Ertl was sent off in the second half.
Whittingham went for broke with his changes but it was the U’s who sealed victory with two further strikes.
The Pompey boss felt weak defending led to his team’s downfall before Ertl’s key dismissal.
He said: ‘We started well and had a lot of success early on.
‘But we have been punished for giving away poor goals.
‘The first goal was a poor goal and the second goal was a poor goal to give away.
‘We caused them a lot of problems but they defended very well.
‘There were positives in our play but you can’t give away goals like that.
‘Then the red card has come along and changed the game.’
Whittingham was clearly unhappy about Ertl’s red card after he caught Oxford midfielder Danny Rose.
An ensuing brawl made a drama of the incident before ref Gavin Ward sent off the Pompey captain.
But Whittingham kept his emotions in check when asked about his thoughts.
He said: ‘The ref said it was a red card, so it must have been a red card.’