The Pompey chairman was hopingÂ supporters wouldn't give him a rough ride for failing to be a substitute at Sunderland last week.
Robert, who was on loan from Newcastle at the time, hadÂ already apologised to fans, insisting he was unable to be a substitute because of injury.
Seapking to The News, Mandaric said: '˜He made a mistake, a silly one, and we understand that.
'˜Let's be united and all get behind him. We need to give him a chance to perform.
'˜We're all emotional and I understand why fans may have reacted as they have '“ they're passionate.
'˜They want players to show passion.
'˜But we really don't want to shoot ourselves in the foot over this.
'˜We have to show we have class and forgive.'
It comes as the man who sat next to Robert in the stand at Sunderland last week backed his claim he had genuine reasons for not being involved in the game.
Midfielder Salif Diao said: '˜There was simply some miscommunication with Laurent and some of the stories that came out afterwards were blown out of all proportion.
'˜I heard one reporter say he had stormed out of the ground, but he was sat next to me.'