Andy Awford refused to defend Dan Butler after his AFC Wimbledon dismissal.
Butler was given his marching orders for a second yellow card against the Dons yesterday.
The left-back flew into a challenge on George Francomb deep in the visitors’ half in the 2-0 defeat.
The inevitable dismissal followed midway through the second half to kill any hopes of a Pompey fightback.
Awford was unhappy with the player for his decision.
The Blues boss viewed the challenge as unnecessary and felt the 20-year-old now has the experience to know better.
Awford said: ‘It’s unacceptable.
‘You can’t be diving into challenges like that when you are on a yellow card.
‘I said to the players at half-time that those who were on yellow cards had to be very careful so we kept 11 players on the pitch.
‘Then Dan has dived in where there is no danger.
‘He’s a young lad but he has experience and played games. He should know better.
‘He flew in. You can’t be diving into challenges like that when you are on a yellow card. Simple.’
Pompey now have a quick turnaround as they go into a testing trip at high-flying Luton tomorrow (3pm).
Awford is pleased there is a quick turnaround after the AFC Wimbledon loss.
He said: ‘It’s a good thing there’s a quick turnaround for us now. I’ve watched them a couple of times and they are a good side.
‘They are strong and we are going to have to be very good to beat them.
‘We will have to be very good to get anything from the game.
‘But there’s no reason why we can’t be. We can beat anyone on our day.
‘We have to go there and get a result because we need to bounce back after the Wimbledon result.
‘I’m no quitter. I will be working and watching this game back. I have to get the players ready to go again.
‘I’ve got a thick skin. I’ve got no problem with the supporters.
‘A few of them might have a problem with me at the moment, though.
‘It’s a results business. We have to keep working as hard as we are and a bit harder.
‘My family will pick me up and give me support.
‘I get that support from those people around me and then I have to pick the players up, get them out there against Luton and get a result.’