AC Milan owner Silvio Berlusconi lambasted his own club and chief executive Adriano Galliani for signing Internazionale winger Mancini on loan.
The Brazilian, who has barely played for bitter city rivals Inter in the last two seasons after a dip in form, joined Milan on the final day of the January transfer window.
"I really don't understand it," the Italian prime minister said.
"He is another creative player when we need someone who can finish off moves. He's not played for two years. I don't agree with getting him and I've told Galliani."
Berlusconi rarely clashes publically with trusted lieutenant Galliani, who has run Milan on his behalf for over 20 years, but often comments on the Serie A club's formations and fortunes.
Leonardo praised Inter, felt Milan didn't move the ball quickly enough and insisted “this changes nothing.”
The Rossoneri played against 10 men for over an hour, but could not break down their city rivals.
“The start was very positive for Inter and they scored an early goal, though obviously the game changed after the red card for Wesley Sneijder.
“In matches like this the little details can be decisive and we did not make the most of the situation.
“Inter have this remarkable strength on the counter-attack and they did it brilliantly tonight. We pinned them back, but the Nerazzurri defended en masse and our possession was a bit too slow.
“We struggled to find the key to unlock them and we tried to spread the play, but Inter closed up tight.”
This extends the gap at the top of the table to nine points, even if Milan have a game in hand to be played in Florence next month.
“Everyone knew Inter were strong, I don't think one match can decide what we can do. The team is maintaining its quality and found an Inter side fired up, who played far better than in recent weeks,” continued Leonardo.
“In the second half we created a lot of pressure, but not finding that goal and then conceding on a free kick made it all the more difficult for us.
“We have always said we're taking it one game at a time and not setting particular objectives. We are second and a game in hand, we are in a very positive position and have the Champions League to look forward to.
“This defeat doesn't change anything for us. This is another stepping stone and one that can teach us a great deal. It must be said that Inter played extremely well tonight, the whole team raised its game.
“We were convinced that we were in good shape, but not that we were favourites.”
Leonardo says “the derby comes at the right time” for a sparkling Milan, but won't discuss their Scudetto chances.
Today's 4-0 drubbing of Siena put them just six points adrift of leaders Inter with a game in hand, the perfect lead-in to next week's Derby della Madonnina.
“The Scudetto? Sincerely I am not superstitious, but I think it best for the team to take it one game at a time,” said the Coach.
“The derby comes at the right time and the fact the two teams are so close in the table puts even more focus on this match.
“It will be a great evening of football and a pleasure to be a part of it.
“We are going through a wonderful period of form, but only time will tell how far we can go. We have the strongest central defensive partnership in Serie A and hope the injury to Alessandro Nesta is not serious.
“Ignazio Abate and Luca Antonini are doing well, so is David Beckham, while Marco Borriello scored an extraordinary goal today.”
Ronaldinho was the star of the show at San Siro with a sensational hat-trick.
“He is giving the fans entertainment and enjoying his football too. When he feels love around him, he gives back with great performances. Ronaldinho has rediscovered the joy of the sport and now he's getting the goals too.”