The pressure has been building on coach Clarence Seedorf following back-to-back Serie A defeats to Juventus and Udinese while the seven-time European champions limped out of the Champions League this week.
Seedorf has played down reports of a falling-out with striker Mario Balotelli, who scored Milan's second after Adil Rami weakened the impact of Cassano's early brace, but the Italy striker continues to flatter to deceive.
To make matters worse, the Rossoneri will be without Christian Abbiati for next week's trip to Lazio after the veteran goalkeeper saw red in the opening minutes.
Amauri and Jonathan Biabiany scored late on to ensure all three points went back to Parma as the Ducale club reached fifth place on the back of a third straight win.
Roberto Donadoni's men are unbeaten since early November and dreaming of a return to European competition, while Milan slipped into the bottom half.
In the fifth minute Abbiati earned a red card for bringing Ezequiel Schelotto down in the area and Cassano did not have to be asked twice to beat early substitute Marco Amelia from the spot.
Milan knew they had to respond quickly or else they would implode and Balotelli elicited gasps when his shot from a corner move rattled a post.
Shortly before the break Philippe Mexes risked scoring a calamitous own goal to further compound Milan's misery, but Amelia managed to push the defender's poor clearance to safety.
Six minutes after play resumed, Cassano made it 2-0 with a second goal against his former club.
The Italy striker, who had almost seen his pass to Afriyie Acquah tucked away moments earlier, took an assist from the Ghanaian and gladly converted his brace.
Enough was enough for the Rossoneri, it seemed, and Rami bit back with a thunderous header from Kaka's cross to drag Milan back into the tie in the 56th minute.
Their tails were up and, when Joel Obi fouled Riccardo Montolivo in the Parma box, Balotelli fired Milan level with a dummied penalty that sent Antonio Mirante the wrong way.
But nothing is ever simple for the Rossoneri these days and the equaliser proved to be a false dawn.
Almost immediately following the restart, Amauri's clever backheel on a Schelotto pass fooled Amelia to hand the advantage to Parma once again.
Hush descended at San Siro and, while most of the Milan faithful began to trickle out of the stadium early, Biabiany confirmed the final outcome with a header that wrapped up another unhappy adventure for the home team.