This is something i've read numerous times over the last few days on here. It's only fair to add a little balance here and there, and it's only fair to point out that that statement is quite ridiculous...
Time for a little history...
He won the Coppa Italia with Fiorentina who were on the verge of bankruptcy, despite not even having the necessary coaching badges to take the job in the first place. The achievement was special considering Fiorentina were a very poor side who had lost it's best players.
He moved to Lazio where he worked under similar financial hardship at a team who were trying to recover from Eriksson's lavish spending spree. Despite Lazio losing some of their best players, Mancini took them to the semi's of the Uefa Cup and the following season he won the Coppa Italia again and led Lazio to the Champions League.
Inter hadn't won a trophy for 15 years. They appointed Mancini. In his first season he again won the Coppa Italia. Inter finished 3rd in the league, and actually only lost 2 games during the entire season. When people on here talk about Mancini sending teams out to 'not lose' they are probably right, but it's an important part of creating a successful mentality. A team cannot go from being inconsistent, to winning every game, regardless of how much money is spent.
He started off the next season by winning the Italian Super Cup, which is the equivalent of the Community Shield. He also won the Coppa Italia for a 4th time. This is the season where the league title was handed to them following the match fixing scandal. Inter again finished 3rd, but were awarded the Serie A title after Juventus and Milan were deducted points for match fixing.
Now the following season is where it gets interesting. There seems to be a misconception that the title was again plainly just handed to Inter, but that isn't true. Juventus were relegated and that in turn allowed Inter to buy Ibrahimovic, but that shouldn't take away from the fact that Inter walked the league, They finished with 97 points, 22 points in front of 2nd place Roma (who weren't deducted any points) and 36 points in front of AC Milan who were only deducted 8 points. During the season Mancini lead Inter on a 17 match winning streak (in the league). Even with Juventus in the league, it's hard to argue against Inter winning the title. Mancini also lead them to the Coppa Italia final where they lost to Roma.
2007-2008 was his last season in charge. Juventus had gained promotion back to Serie A. Inter won the league again finishing 3 points ahead of Roma, despite holding an 11 point lead halfway through the season. Form dipped a little, but picked up again in time for them to win their 3rd title in a row. Again there was no points deductions for any of the top teams.
Mancini again lead Inter to the Coppa Italia final where they again lost to Roma.
The reason Mancini was sacked was down to their European form. In each of the 4 seasons they made it out of the group stages, but lost in the knockout rounds to AC Milan, Villareal, Valencia and Liverpool. He reached the last 8 twice.
All in all Mancini lost a total of 13 Serie A games out 142. He took them to the Italian Cup Final in each of his seasons there. He won it twice with Inter, and also won it with Lazio and Fiorentina. He broke the record for consecutive wins in a top 5 league in Europe by leading Inter to 17 straight league wins. Inter also went just over 1 year unbeaten in the league with Mancini in charge. He has never failed to qualify for Europe in his 9 seasons as manager.
He's 45 years old. He has 3 Scudetto's (2 of which were won outright...regardless of point deductions). He has 4 Coppa Italia's and 2 Italian Super Cups. He also has 2 Coppa Italia runners-up medals and two Italian Super Cup runners up medals. He's also qualified for the Champions League with two very different teams. He's more than qualified to make us his 3rd.
Saying he is out of his depth is not only factually incorrect, it's also quite daft. This man will lead us to glory. We need to be patient.