Michael Appleton admitted his men collapsed under the survival pressure as they were battered by Burnley.
Pompey gave themselves a mountain to climb to maintain their place in the Championship as they fell to a crushing 5-1 loss against the Clarets on Saturday.
The Blues are now five points from safety with six games left.
The hope was the Great Escape could gather pace with victory over a side who were winless in seven.
But Pompey wilted under the expectation and were demolished by four second-half goals.
Appleton said: ‘There was a lot on the game.
‘If we could win the game, there was a feeling about the whole camp we could really go on and win a lot more from now until the end of the season.
‘Probably the pressure that came with the game – and considering how big the game was – really affected the players in the second half.
‘I thought a few of the players lost their discipline and they lost out on their personal battles, which was disappointing,
‘It’s difficult sometimes to try to put your finger on why exactly that happened.
‘A lot of it comes down to the level of expectancy.
‘There was a little bit of pressure on the game and we didn’t deal with the disappointment of going down to that goal in the first minute of the second half.
‘They’ve got to expect that as footballers.
‘We all expect to win games and it was a game we were more than capable of winning, as we proved at their place earlier in the season.
‘It was a major disappointment.’
Pompey now have a week to lick their wounds before the south coast showdown with Southampton.
The Blues will be underdogs against the league leaders.
Appleton believes that can work in his team’s favour, though, with form likely to go out of the window in the heat of a derby day battle.
And he is trying to take the positives from Saturday’s loss as he begins preparations.
He said: ‘We have got a week to do things and try to put it right.
‘Whether the expectations have been dramatically lowered after the game, maybe that might help us next week going to Southampton.
‘A lot of football is psychological, you only have to look at the way they performed and the way we performed in the first half.
‘They were never really in the game yet got a goal from it, while we only scored one.
‘They were absolutely clinical in terms of their finishing.
‘We have won games together, we have lost games together and been through a lot together over a short period of time.
‘We will take the defeat on the chin.
‘You get beat five and you probably look back at it and think they must have dominated the game and dominated the game for long, long periods.
‘But it was probably as good a performance by us in the first 45 minutes as it was against Hull.’