'Il Talentino' comes on to set up the winner as Milan extend their lead in Serie A.
Rodney Strasser netted the winner after fellow substitute Cassano had run at the wilting defence in the 85th minute to send Milan five points clear in the Scudetto race.
Milan were the first to threaten when, after a period of sustained pressure in the opening 10 minutes, Robinho got away down the left and controlled before attempting to send an angled shot inside the far post, but Michael Agazzi got down well to push the ball wide.
After their uncertain start, the Isolani began to finally see some of the ball and were unfortunate not to take the lead midway through the first half when defender Michele Canini rose high above Thiago Silva to head goalwards, but with Christian Abbiati beaten the ball came back off the foot of the far post.
Alessandro Matri seemed to be brought down on the edge of the box as the interval approached, but the referee waved play on, and on the stroke of half-time Alessandro Agostini had a long-range effort punched away by Abbiati, with Matri just unable to reach the rebound.
Milan should arguably have taken the lead in the sixth minute of the second half when Alexandre Pato broke the offside trap bearing down on goal, but Agazzi did superbly to stay big and force the Brazilian to make the first move before pulling off a stunning sharp save with his left arm to foil the Rossoneri.
At the other end, Pato was again denied when Daniele Conti blocked superbly as the striker turned on Merkel’s low cross from the right. And before long, Robinho wasted a great opportunity on a two-on-one break by displaying the heaviest of first touches to allow Agazzi to collect with ease.
Massimiliano Allegri finally threw on new signing Antonio Cassano for the final quarter of an hour, and ‘Il Talentino’ could quickly have picked up an assist had Robinho been able to keep his shot down after collecting Cassano’s header and setting himself.
But ‘Fantantonio’ wasn’t to be denied a share of the headlines on his debut, as he forged the opening which allowed substitute Rodney Strasser to fire home an 85th minute winner to stun the home crowd.
The defeat means Cagliari remain within reach of the drop zone, with 20 points from their 18 games, while the Rossoneri extend their lead at the top to five points thanks to Lazio's draw, though a win for Napoli at Inter tonight would see the gap cut to three once more.