Robbie Neilson felt his side paid the price for letting Pompey gain a head of steam.
The MK Dons boss saw his team well beaten as they fell to a 2-0 loss at Fratton Park.
Neilson believes his team took a backward step after shipping two first-half goals as the Blues dominated.
The 37-year-old reckons his players didn’t rise to the occasion of playing at Pompey’s home.
And that led to a day the Scot wanted to quickly put to bed.
Neilson told the MK Citizen: ‘The quality was miles off where it needed to be, but we have to take it on the chin.
‘We had opportunities to pass the ball better but we didn’t. It is up to everyone to improve. On Saturday, we took a backwards step.
‘People make mistakes – that’s football. Yes it had a big bearing on the game, but it came down to the performance on the whole and it wasn’t good enough.
‘We started alright, the quality into Osman Sow was good but we didn’t link it well enough and the quality in the attacking area was really, really poor.
‘It allowed Portsmouth onto the front foot, we gave away a goal and it spiralled from there.
‘We had words at half-time but it just wasn’t good enough. It’s a fantastic place to come and play, but we didn’t turn up after the first 15 minutes.
‘The second goal is always key. It was just a bad day all round.
‘The big thing about this league is being resilient.’
MK Dons were on a run of one loss in nine before being hit by two defeats on the bounce.
The challenge now is for them to return to that form as they face Walsall on Tuesday night..
‘It’s hard to put together a long, sustained winning streak,’ Neilson added.
‘We managed to do it but the past couple of weeks have been disappointing.
‘But we’ve got the chance to get back on the horse on Tuesday night.
‘We have to go again Tuesday and make sure we put in a better performance and take three points.’