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 suspen...