Michael Appleton has admitted Scott Allan was not mentally prepared to face Notts County after he was involved in a car accident.
The Pompey midfielder was sent off on Saturday as his side slipped out of the FA Cup at the first-round stage.
That was hours after a man was left in critical condition in hospital after being struck by a car driven by Allan.
Allan was not arrested. The police are expected to question him again, though.
And Appleton believes the incident affected the 20-year-old against the Magpies.
Allan told the Pompey boss he was prepared to play Saturday’s game.
But he was sent off for a rash challenge on Andre Boucaud in the second half as the Blues tumbled to a 2-0 defeat to the Magpies.
Appleton said: ‘There was an incident on Friday night.
‘I don’t want to comment too much on it because the authorities are looking into it.
‘I did ask Scott before the game if he was mentally in the right place to play.
‘Obviously he wasn’t but I can only ask for honesty from my players. I can’t really do any more.
‘He said he was fine and it wasn’t a problem at all – but you learn.
‘Possibly it reflected in what happened (the red card).
‘But anyone could have been sent off.
‘It was that kind of scrappy game.’
Confusion reigned over whether Allan was handed a second yellow card or straight red after the tackle on Boucaud.
Referee Darren Sheldrake indicated it was a red in the wake of the game – meaning a three-game ban for Allan.
The Scot’s level of performance on Saturday dropped off from his hugely impressive return against Sheffield United last week.
But Appleton decided against taking him off with the belief he could deliver one moment of match-changing quality.
The Pompey boss explained he would now speak to Allan in an effort to get him back on track.
Appleton said: ‘He’ll be told in no uncertain terms about that (having a quiet game).
‘We’ll speak to him and make sure we address that.
‘Hopefully that will be a positive because sometimes things like that have to happen with talented players.
‘If that brings the best out of him, hopefully we can turn a negative into a positive.
‘Sometimes you can see it with less-gifted players who aren’t playing well. But it’s difficult.
‘In the back of your mind you are thinking Scott can do something special. It’s similar with Akos Buzsaky.
‘You try to keep those players on the pitch because they can hurt the opposition.’