Paul McCallum’s desire to impress overspilled into seeing red for Pompey, according to Andy Awford.
The on-loan striker has been given an automatic three-match ban following his Southend dismissal.
In addition, he has pleaded guilty to a Football Association charge of improper contact.
That relates to his actions when leaving the pitch following the sending off, with contact made with referee Lee Collins.
McCallum’s ban is likely to total a minimum of four games, starting with tomorrow’s trip to Wycombe.
Awford has revealed the 21-year-old has shown remorse for his actions.
And Pompey’s boss insists the youngster’s desire to do well in front of the Fratton faithful was his downfall.
He said: ‘Paul has admitted the FA charge.
‘I have watched it back and there is no getting away from it – he deserved to be sent off the field of play for grabbing their player, without a shadow of a doubt.
‘It is out of character for Paul. He has got caught up in trying to impress.
‘He had gone on as substitute, we’re getting beat and he’s not had many experiences of playing in front of 15-16,000 crowds.
‘I just think he got caught up in the game, where he was trying to impress me, wanting to start the next time and was desperate to try his best.
‘Paul is full of remorse, he’s upset. I have spoken to him and dealt with that internally, he feels he has let us and himself down.
‘He is going to get three matches and possibly an extra little bit – we don’t know what that is going to be yet.
‘He has grabbed their lad, simple as that, you can’t do that, there are no grumbles about his sending off.
‘It’s what happens now moving forward in terms of he didn’t leave the pitch as he should and the authorities will look at it.
‘It is the first time it has happened to him, I have done all the due diligence and the homework before taking him and it is not something anybody has flagged up.’
McCallum will now miss tomorrow’s Wycombe trip, Hartlepool (February 7) and Dagenham & Redbridge (February 10).
Although Awford will not be sending him back to West Ham early.
He added: ‘He’s a contracted player, we cannot just say to West Ham “rip that up”, it is no different to any other player.’