Steve Cotterill pinpointed Dave Kitson's red card as the turning point in FA Cup defeat at Brighton.
The Blues' season hit a low at the Withdean Stadium as Cotterill's men fell to a 3-1 loss.
Pompey could not complain with the outcome of the game with the Seagulls dominant.
Cotterill is confident it would have been a different story, however, if Kitson didn't walk for an altercation with Adam El-Abd.
He said: 'We started very brightly, I thought.
'But the sending off was a definite turning point.
'I thought we dominated before that.
'I would have been very happy if I'd kept that 11, because if we'd kept that 11 we'd have won the game.'
The three goals conceded made it a woeful 12 goals shipped in four games for Pompey.
Cotterill was frustrated with defending by his team.
He said: 'We lost the marker on the first goal. I'd have to see the second goal again, but it looked like a penalty from where I'm standing.'