The Brazilian midfielder has been back in training ahead of the Wembley showpiece, but Mendy, who has suffered a setback since recovering from a knee injury, has been ruled out by Pep Guardiola.
“Benjamin Mendy has not been selected because he’s not fit,” said the City boss.
“He will be ready next season, but he is not available for the final.
“Fernandinho trained today and yesterday.”
Having retained the Premier League, Guardiola’s side have been installed as strong favourites against the Hornets.
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Given City’s form over the past two seasons, it’s not something the Catalan finds surprising, but he has not allowed his team to rest on their laurels and issued a warning over the threat posed by Watford.
“It is what it is after what happened last season,” he added.
“We won the most incredible Premier League ever, in terms of results and competitors. We are incredibly satisfied.
“We celebrated for a short time and were back to business in the last few days to focus on Watford because I know how difficult it was in the home game, with the the quality up front with [Roberto] Peyreyra, [Troy] Deeney and [Gerard] Deulofeu.
“The long balls to Deeney are always unstoppable, even with our tough central defenders. Maybe he is the best striker for setting the ball to his mates.
“We handled the defensive side really well, with set pieces they have Holebas who is an incredible taker.
“We spoke about the players and hopefully we can be focused for one more day, to compete again to be champions.”
Watford stand between City and yet another milestone, as victory on Saturday would see Guardiola’s side become the first team to win all four English trophies in one season.
It would be a quite remarkable achievement, but as he did before the Brighton game, Pep was keen to highlight that a loss would not take anything away from his players’ level of performance.
“We could not win, but the effort would have been the same,” he explained.
“For one line you can win the league or not win the league, or you can win the FA Cup or not win.
“The important thing is to judge what we have done that season. To take back to back Premier Leagues, to be in the FA Cup final and to win four titles in one season would be incredible.
“But honestly, it won’t change my judgement of the team if tomorrow we don’t win.”