Erik Huseklepp was distraught after his stoppage-time own goal, while Blues keeper Jamie Ashdown also shouldered his share of the blame and apologised to his team-mates at half-time after his horrible mistake handed Peterborough an early lead.
But Mullins revealed that the dressing room had already rallied around the pair, rather than pointing the finger of blame at anyone.
Mullins said: ‘Jamie is an important part of our squad.
‘Against Blackpool he made two or three important saves for us and we don’t forget that.
‘He kept us in the game to help us with that win.
‘This time he made a mistake, but everyone does.
‘Unfortunately for him, if he makes a mistake, it ends up in the back of his net so it’s hard for him.
‘But all the players are behind him and he knows that.
‘That’s the goalie’s lot. He’s the last line of defence and if he makes one mistake it ends up in the back of the net.
‘He came in at half-time, held his hands up and apologised but we all got behind him.’
Mullins and the other senior professionals also had words of comfort for Huseklepp, who was left in a miserable heap after his attempts to prevent Peterborough’s late breakaway instead saw him put through his own net.
But ex-West Ham midfielder Mullins appreciated the Pompey fans chanting the Norwegian’s name in support at the final whistle and believes that will have been a major comfort after the forward turned in an otherwise impressive display.
Mullins said: ‘Erik was telling us this week how he feels like he is getting the pace of the game and then that goes and happens to him.
‘All the players are behind him and we told him to get his head up, keep creating goals for us and the fans helped with that at the end as well by singing his name.
‘It was a nice touch from our fans and I’m sure that will help him because you can tell that hurt him.’
Mullins, meanwhile, was at a loss to explain the defeat and believes it was right that Pompey were still going for the win in the dying seconds.
He said: ‘They hit us on the break from our corner and it ended up in the back of our net, which shouldn’t happen.
‘But in the last minute, what are you going to do when you have a corner?
‘You are definitely going to go for it and that’s what we tried to do.
‘Once we got back into the game, we had loads of the ball, had chances, goalmouth scrambles, one-on-ones but we didn’t take those chances, unfortunately.
‘I thought we were definitely going to go on and nick it so it’s a real killer to lose like that.’