Leon Barnett apologised to referee Simon Hooper and City boss Paul Lambert for his 'out-of-character' reaction to being sent off on Saturday.
Barnett was dismissed for a second yellow when he brought down David Nugent deep into injury time, handing Pompey a penalty which gave them a 2-0 win.
As Barnett walked past referee Simon Hooper, he threw the ball at him - completing a miserable day for the 25-year-old defender, a day after agreeing to turn his loan move from West Bromwich Albion into a permanent deal in January.
But Barnett immediately regretted his actions and made his apologies.
'I went in straight away after the game to the referee and the gaffer to say sorry for my actions,' he said. 'I knew immediately that what I did was wrong.
'What happened was out of character for me and I wanted to apologise for doing what I did.'
Barnett will miss the trip to Coventry - where he spent time on loan last season - but could face more punishment, depending on what the official includes in his match report.
Barnett's pre-match tally four yellow cards remains.
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