Milan are now sitting comfortably inside the top four after a difficult start to the season.
Galliani said: "He's the key to everything. This is not the AC Milan of (Carlo) Ancelotti, now it's the Milan of Leonardo. In fact right now we play with 4-2-4. This system is also liked very much by President Berlusconi.
"We are finally able to propose again to play with four strikers, like Ronaldinho, Pato, Borriello and Clarence Seedorf: We wanted time, but now he has changed our spirit."