Leonardo replaced Rafael Benitez two weeks ago, despite managing Milan last season, and soon started to win over the Inter fans with Thiago Motta firing home after three minutes in an entertaining 3-1 home win over third-placed Napoli.
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Inter, without suspended striker Samuel Eto'o and injured duo Julio Cesar and Wesley Sneijder, buckled midway through the first half when Michele Pazienza nodded in a corner but superb headers from Esteban Cambiasso and Motta sealed matters.
"After a tough win, above all with a goal from a lad from the youth team, I think there's reason for the club to be satisfied," Milan coach Massimiliano Allegri, whose side next host Udinese on Sunday, told Sky.
"Definitely Cassano is not in top condition, it's enough to look at him, but it's normal as he has not trained for two months. He is already more in form than he ought to be."
Milan were much changed because of injuries and Zlatan Ibrahimovic's suspension but teenage debutant Alexander Merkel made a bright start alongside returning striker Alexandre Pato.
Allegri's ex-club Cagliari, now bossed by former Milan stalwart Roberto Donadoni, hit the post through Michele Cannini.
The home side's woes began after three minutes when in-form forward Fabio Quagliarella was carried off with a knee problem, further highlighting the club's need for a new striker.
