Mancini has left open the door for new signings at City in January - but claimed Rooney would have a far harder job crossing the divide from Old Trafford than Carlos Tevez had last summer.
Rooney seems to have slammed the door Sir Alex Ferguson claimed he had left open to his United career with a statement on Wednesday evening that cited the Red Devils' lack of ambition for his decision to quit.
It is an accusation that you certainly could not level at City, who have spent huge sums of Sheikh Mansour's vast wealth in rapidly climbing the Premier League ladder.
The arrival of Tevez last summer remains their signature capture, prompting those award-winning "Welcome to Manchester posters" and getting under the skin of so many United supporters.
It hardly needs to be spelt out they are the one club in the world who would have no trouble finding the money United would want for Rooney, a player Mancini recognises as a world-class talent.
But, according to the City boss, Rooney would have far more baggage to haul across Manchester than Tevez.
"Rooney is a good player, and is like Carlos Tevez for us," said Mancini.
"If they lose him for next year, they will lose an important player.
"In December it is normal that teams that want to improve and have money to spend, will want to buy players.
"Maybe in January we can change some players, but it depends on the situation. If we have injuries, maybe.
"It was different for Carlos. Rooney is English and has played for United for a long time, Carlos was there for two years. That makes a difference."
Although Mancini was commenting on the situation prior to Rooney's damning assessment of the direction United were heading, the Italian still expects the former Everton star to remain at the club where he has won three Premier League titles - and the European Cup.
"Rooney plays for United and has another year on his contract," he said.
"Every time, when I was in Italy, and there was a problem between the best player on the team and the manager, the coach or the owner, in a month the situation changed and he signed a new contract."
Rooney's assessment of the situation has cast doubt on that.
Yet even before a scarcely believable four days, which started with hints that Rooney was ready to abandon the United cause, the perception has been of a seismic power shift in English football, and the epicentre is undoubtedly Manchester.
"I don't think any club can win for 50 or 100 years," said Mancini.
"City must now work well to try to win something this year.
"At that moment the situation can change. And that is what we want."
Mancini very carefully avoided the subject of Rooney joining City, or whether he would be interested in buying him.
But the Blues boss made a pretty forceful point, which emphasised why the Eastlands outfit must at least take more than a mere passing interest in current events at Old Trafford.
"I don't feel sympathy for Sir Alex. This is football," said Mancini.
"Football has changed, and every player can change club every year - it's not like 10 years ago.
"There are not a hundred fantastic players in the world - there are maybe 15 who can change a squad.
"If these players have this chance, it is normal."