Wayne Rooney said that Manchester United had shown their neighbours City who was boss in their 3-2 Community Shield comeback victory, handing them what he described as a "footballing lesson" which set down a marker for their Premier League title defence.

The striker, who was central to Nani's wonderful equaliser as a youthful United turned round a 2-0 half‑time deficit, revelled in his team's superiority and the bragging rights that accompanied the result. Nani, the man of the match, scored the winning goal in the third minute of injury time.

"This shows who the best team is," Rooney said. "All game we dominated. The difference the young lads made was outstanding. We never know when a game is finished. We took them apart. The scoreline is deserved. We're champions and we're the team to beat. We want to prove that."

Rooney was asked whether the victory tasted sweeter because it had come against City, who defeated United at Wembley in last season's FA Cup semi-final, a tie that the striker missed through suspension. "Of course," he replied. "Obviously they are wanting to try to fight for that title and today I think there was only one team that was going to win. We dominated the first half and they had five minutes when they scored two goals from nowhere but we showed our character to come back and win."

Rooney later tweeted to his million‑plus followers: "Have to say. I think today was a footballing lesson. Great win for the champions."

Sir Alex Ferguson, the United manager, felt that the most pleasing aspect of the game was the way that his young players showed their mettle. He made three half‑time changes, sending on Phil Jones, Jonny Evans and Tom Cleverley, who was outstanding, and when he replaced Patrice Evra with Rafael da Silva in the 71st minute his team's average age was 22. Dimitar Berbatov came on for Danny Welbeck in the last minute to alter the statistic slightly.

"For us I think it just confirms what I've thought," Ferguson said. "People were saying that we're not the best United squad and things like that but you've got to remember a lot of young players will improve. We are very confident with this group of young players."

The blot on United's afternoon was provided by David de Gea, the goalkeeper who was signed for £18.3m from Atlético Madrid. He might have come for the David Silva free-kick from which Joleon Lescott glanced City ahead while he was horribly slow to get down and across to Edin Dzeko's 25-yard blast that made the scoreline 2-0.

"City are a big team and the delivery of the ball [for the first goal] was good ... goalkeeper no chance with that," Ferguson said. "The second goal I thought we could have closed them down but the shot swerved a bit and just caught the goalkeeper on the wrong foot. But I think that you have to cope with these things and he's [got] no problem."

The City manager, Roberto Mancini, who made his players watch and applaud as United lifted the trophy, faced more questions after the game about Mario Balotelli. The maverick forward had walked straight off down the tunnel when he was substituted in the 59th minute, although he did return to the bench. Gary Neville, in the Sky Sports studio, called him an "embarrassment" to City.

"None of my staff told Mario to come back," Mancini said. "Every time he does something, people read things into it. He can play better than he did today."

City needed to show a "strong mentality", Mancini said. "I think that they played better than us but, when you go 2-0 up, you need to control the game. Maybe at the end the correct result would have been 2-2 but this is football. United is a top squad ... it's better than any other team. But we can win the title."

He denied that City had an interest in the Internazionale midfielder Wesley Sneijder, who is coveted at United, but he hoped to make further signings before the end of the month. Arsenal's Samir Nasri is prominent on his wish list. "We need to complete our squad. We have the targets we had at the start of pre‑season. Sneijder said yesterday: "All I know is that Inter need money and apparently I'm for sale for the right amount. I have spoken with neither Manchester City nor United so I cannot say anything about my future."

Ferguson said Danny Welbeck had picked up an ankle injury and the striker can be expected to be withdrawn from the England squad who face Holland at Wembley on Wednesday.