City, fresh from celebrating their historic fourth successive Premier League title, face a United side sore from finishing way back in eighth place at Wembley on Saturday.
Chasing back-to-back domestic doubles and a repeat victory over United after last season's success, City are overwhelming favourites.
Yet Guardiola is taking nothing for granted.
The City manager said: "People think it's going to happen because we finished 31 points in front of United, people say it will be easy.
"I understand that but it's a different competition, one game. It can be 10 against 11, bad decisions, mistakes, whatever - and you can lose a game. We know we can lose a game.
"In long periods, we have been better than United these past years, it's obvious. It's facts - every game we are there.
"But in one game, anything can happen. The players know it and feel it. So I hope we are ready to play the last game of the season and to try to win the FA Cup.
"It's the FA Cup. Wow! It's nice. That's why it deserves all of our focus."
Guardiola has now won 17 trophies since taking charge at City in 2016.
Despite success becoming the norm at the Etihad Stadium, he maintains there is nothing routine about it and there are still nerves before big games.
He said: "I cannot deny that the mood in the offices is really high, it's really good. People laugh a lot.
"I don't want to be concerned too much because I want the players to enjoy, to celebrate the fact they won the Premier League.
"But we will have the butterflies, wondering what they do and what we can do, the decisions to take, how the players will feel. All of those questions you have before a game is going to happen."
The mood is different though at Old Trafford where Erik ten Hag insists his job is secure despite a report saying he will be sacked.
After winning the Carabao Cup, finishing third in the Premier League and reaching the FA Cup final during a promising first season, things have gone awry under the Dutchman this term.
United are preparing to face Manchester City in a second straight FA Cup final but stumbling home eighth - their worst Premier League finish - has led to intense talk about Ten Hag's position.
The under-fire boss brushed aside any suggestion of the final potentially being his last match in a build-up that saw him asked whether talks were planned with the owners early next week to reflect on everything.
"No, we already did (it) and on Sunday I will go on holiday," he said. "I think I deserve it."
Asked then whether he has been given any assurances he will definitely be United manager next season, he said: "It is not necessary. We go for next season."
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