Manchester City skipper Kompany saw red in Sunday's 3-2 defeat to Manchester United at the Etihad Stadium.
Kompany will learn later on Tuesday whether he will serve a four-match ban for his tackle on Nani, which Sir Alex Ferguson felt was dangerous.
Mancini still feels his captain did nothing wrong and would be unhappy if the verdict went against the Belgium international.
"My opinion has not changed," he said. "I am confident because Vinny only played 10 minutes.
"He was sent off for nothing. He has already missed one game.
"I don't think about this (being banned). It would be not correct if that happened."
Mancini also claimed he was right to point out that Wayne Rooney gestured to referee Chris Foy for Kompany to be sent off, even though the United star claims to only have been pointing out it was a two-footed tackle.
"I joked with Wayne Rooney. But he did it," said Mancini. "Nani was there. He didn't say anything."
If Kompany fails in his appeal his will start his suspension by missing Wednesday night's Carling Cup semi-final first-leg with Liverpool.
Kompany would also miss the return and, in between, crucial Premier League games against Wigan and Tottenham.
It would merely heighten a personnel problem that could leave Mancini without six senior players for Wednesday's game, including Kolo and Yaya Toure, who are on African Nations Cup duty with the Ivory Coast.
In addition, Edin Dzeko and Mario Balotelli are both major doubts, along with star midfielder David Silva, who was replaced at half-time on Sunday.
Meanwhile, Mancini confirmed there had been no movement on the potential transfer window departures of Wayne Bridge and Nedum Onuoha.
And, once again, the City boss said he hoped for a speedy conclusion to the Carlos Tevez saga, with Inter Milan joining city rivals AC in trying to tie up a deal for the controversial Argentina forward.
"I hope there will be a good result for him in January because he has not played for three months."