The Argentine came on for the last few minutes of Tuesday's 1-0 win at home to Tottenham Hotspur which guaranteed City their first top-four Premier League finish and a chance to show off their expensively assembled team in the Champions League.
"Carlos is getting better yesterday, we have another two days to decide whether he can play or not," the Italian told a news conference.
"We will decide this on Saturday morning for Carlos."
The club's top scorer this season, and often their most influential player, had been sidelined for a month with a torn hamstring.
With two league games left, they could still finish higher than fourth to bypass the Champions League final qualifying round and go straight into the group stage of Europe's elite competition. But thoughts of that will take a back seat on Saturday.
"We did a piece of history for Manchester City," said Mancini, adding that with the main goal of the season out of the way it would be easier to turn their attention to the FA Cup.
"Now we should keep our concentration because we have to play this important game, the final. If we win two (league) games, maybe we can finish in third position."
City, who won their fourth and most recent FA Cup in 1969, are hoping this season will be a springboard to even more prestigious things and Mancini said it should now be even easier for the world's richest club to attract top players.
"It will be easier to get good players because all the good players want to play in the Champions League," he said.
"I will focus on my game, not about United," said Mancini.
"I can understand that the FA Cup is very important. It's probably not good to play all the games on the same day," said Mancini. "For me it is the first time, for Manchester City it is the first time in many years (that they are in a final). I think it will be a good day."
