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.
Seventh-placed champions Inter have two games in hand but are 13 points behind Milan, who extended their advantage at the top to five points with Lazio in second drawing 0-0 at Genoa.
Cassano, bought from Sampdoria to replace the soon-to-depart Ronaldinho, came on in the second half and played in Sierra Leone youngster Rodney Strasser for a vital 86th-minute winner which had a hint of offside.
"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.
ROMA FOURTH
Juventus slipped eight points behind Milan in the race for the scudetto after on-loan Parma winger Sebastian Giovinco scored twice in a shock 4-1 success at his parent club.
The defeat was the first in 19 games in all competitions for fifth-placed Juve, who were down to 10 men when Felipe Melo was sent off after 17 minutes for kicking Massimo Paci in the face.
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.
Giovinco struck either side of the break before Nicola Legrottaglie halved the deficit with a header.
However, Hernan Crespo immediately secured victory from the spot after being fouled by Giorgio Chiellini on 61 minutes and former Juve man Raffaele Palladino scored on his debut late on.
AS Roma moved above Juve int
