Napoli remain in third but their third defeat of the campaign, and second consecutively following a 3-0 reverse at Juventus a fortnight ago, has given leaders Roma and second-placed Juventus an unexpected title boost.
Ahead of Sunday's fixtures Roma, who host Cagliari late on Sunday, hold a one-point lead over champions Juventus who can stretch their lead over Napoli to six points away to Livorno earlier on Sunday and possibly go top of the league.
Napoli's defeat has also given incentive to Inter, who sit fourth at three points adrift of Rafael Benitez's men.
On paper, the international break was not kind to Napoli.
Desperate to make amends for their reverse in Turin, the hosts dominated throughout but ultimately failed to hit the net, the first miss coming early on when Antonio Mirante blocked Gonzalo Higuain's angled drive from close in.
Lorenzo Insigne sent a curling shot wide and then Higuain missed the target again on the half hour to leave the hosts frustrated at the break.
Napoli dominated the early exchanges after the interval and had the ball in the net thanks to Higuain on 70 minutes only for it to be ruled offside.
Minutes later, Cassano silenced the San Paolo after being allowed to run down on goal and send a low angled strike past the hands of an outstretched Pepe Reina.
Napoli sniffed a leveller in the 89th minute but Belgian substitute Dries Mertens' long-range free kick smacked off the crossbar.
Milan had been looking to claim their first league win since a 1-0 win over Udinese in mid-October and took a fourth-minute lead thanks to Kaka.
It was the Brazilian's 97th goal for the club, but Milan's joy was short-lived.
The Rossoneri were given a chance to restore their lead just after the half hour when Mario Balotelli, looking for his first league goal in two months, was hauled down by Thomas Manfredini whilst about to shoot.
Manfredini was shown a straight red card in the 36th minute but Balotelli's soft spot-kick was easily collected by Mattia Perin in the visitors' goal.
Milan put their man advantage to good use in a dominant second half and came close in ten thrilling, closing minutes but had to settle for a fifth draw in 13 games which left them 18 points off the lead and 14 behind the first UEFA Champions League qualifying place.