The Romanian goalkeeper has played in all of City's domestic cup fixtures this season, and Mancini believes it would be unfair to alter his selection policy for the Wembley showpiece.
Mancini confirmed Joe Hart is hoping to play, but when asked if he would select the England international, the City manager said: "I don't think so.
"We decided this at the start of the season. Pantilimon deserves to play.
"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."
Meanwhile, Mancini was guarded when asked if a striker was on his summer shopping list, insisting his focus is on the remainder of the current campaign.
"We can't know which player can score 25 goals next year. If we knew that we would be magicians," Mancini added.
"We have good players and were unlucky this season. Now it is important we win the FA Cup and then try to improve our team."