what Salvatore said - he is not a great one.
Mancio is decent enough to win you a domestic title provided his team outspends the opposition 5-10/1 and that opposition is either too weak or has a horrendously inconsistent season, including many injuries, so they end up gifting you the title (8pt lead, 5 games before the end = a title thrown away) ... that was the case with Inter in Serie A and this year in the EPL too.
However, if City are looking for a coach to take them to the next level (as in win them titles in a tough, competitive league and/or be contenders at the highest level in the CL), he is not the right man for the job and they should look elsewhere ... that's what Inter realized after 3 years and it's what City will realize too after 1.5-2 bill and 2-3 years later.