The England striker says his side showed "who the best team is" in their Wembley win and claims the FA Cup holders were dominated throughout as he sets sights on league title
City took a two-goal lead into the interval thanks to goals from Joleon Lescott and Edin Dzeko, despite United having dominated possession.
However, a second-half fightback culminated in United winger Nani scoring a 93rd minute goal that saw United take the spoils with a 3-2 win, and Rooney feels it was just what they deserved.
"This shows who the best team is - all game we dominated," Rooney told BBC Radio 5 Live.
"They had five minutes where they scored two goals, and that was about it.
"The scoreline is deserved because of our passing, and the brilliant chances we created. We are champions and we are the team to beat - we need to go out and prove that.
"Full credit to the whole team, that's our attitude. We never know when a game is finished. We hope title number 20 will come. But it's a long season and there's a lot of good teams in the league. Our main aim is to win that title again."
However, Rooney was not done with his assessment of the match, later using his Twitter account to declare: "I think this was a footballing lesson."