The Romanian keeper has played in all of City's cup games so far this season and the Italian believes it would be unfair to revert back to his regular No.1 instead for the final.
The 26-year-old, who stands at a towering 6 feet 8 inches, has conceded just once in the FA Cup so far this season, with Chelsea striker Demba Ba in the semifinal the only man to have scored past him.
When asked if Hart would play on Saturday after Saturday's 0-0 draw against Swansea City, Mancini told reporters: "I don't think so. We decided this at the start of the season. Pantilimon deserves to play."
He added, "Joe won the FA Cup two years ago, he won the Premier League last year, he is the best keeper in England and I think he should be happy his second keeper can play in a big game.
"I know he wants to play but we decided at the start of the season it was important for Pantilimon to play in the two cups."