The substitute came on to devastating effect, firing in the Rossoneri's clinchers as his team maintain pace in the upper echelons of Serie A
Fiorentina had ousted Massimiliano Allegri's men from third spot after their 3-0 win away to Sampdoria earlier on Sunday, ensuring San Siro carried an air of trepidation for the game against the Elefanti.
Stephan El Shaarawy was reunited with Mario Balotelli up front after the latter returned from suspension, with Antonio Nocerino and Marco Amelia also handed rare starts as Milan re-mounted their bid for Champions League qualification next season.
Within two minutes, the hosts had appealed for what they felt was a cast-iron penalty for a hand-ball by Nicola Legrottaglie in the penalty area, although the referee waved protests away.
Mattia Di Sciglio's yellow card for an over-zealous challenge down the flank typified Milan's sense of urgency, with Riccardo Montolivo becoming the first to test Catania's stand-in custodian Alberto Frison with a daisy-cutter which needed saving with some difficulty.
Balotelli then arrived the fraction of a second late to tap-in Kevin-Prince Boateng's tantalising low cross, as the Rossoneri continued to up the pressure.
But it was Catania who took the lead against the run of play. Legrottaglie rose highest in the area to meet Francesco Lodi's perfect free kick, in what was the visitors' first attempt on target. Suddenly, the Sicilians were all over Milan. Alejandro Gomez burst forward with intent and Amelia had to push away his fierce piledriver at full stretch.
Sharply stirred by the shock of conceeding, Milan swarmed back at their opponents. Boateng hit the crossbar with a stunning first-time volley, then Nocerino's follow-up had to be parried by Frison. The 25-year-old keeper next continued his fantastic duel with Balotelli, turning the striker's menacing free kick around the post.
Alas for Frison, even his heroics could not deny Mathieu Flamini, the Frenchman pouncing on a rebound to emphatically tuck home a fierce angled drive on the stroke of half-time to take the wind out of Catania's sails.
The restart afforded Rolando Maran's embattled shot-stopper no respite, El Shaarawy keeping him on his toes with a few attempts from the edge of the box, before the Italy international took a tumble in the box which failed to convince match official Davide Massa.
Montolivo was denied at point-blank range but, for all their endeavour, the European wannabes were punished for another defensive lapse when a static rearguard failed to pick off a slide-rule pass for Gonzalo Bergessio, who seized his opportunity to exquisitely chip Amelia.
El Shaarawy's subsequently scuffed his strike from a good position to sum up his side's woeful night in front of goal, and just as the crowd begin to acknowledge their team returning to the dressing rooms on the back of defeat, substitute Pazzini produced the simplest of tap-in's to bring San Siro to its feet in celebration.
The 28-year-old, benched in the wake of Balotelli's return, fully justified his introduction by firing Milan ahead for the first time all night. Gomez's cries for a foul were ignored as a raking clearance found its way to El Shaarawy. Breaking past his marker, the Milan top scorer was denied by Frison, but Pazzini pounced on the rebound to rifle in the winner.
Milan were not yet finished, and Balotelli picked himself up after being brought to a crude halt in the Catania area to rifle home from 12 yards. Full-time prompted wild celebrations as the scale of their feats sunk in on the team, as they leapfrog Fiorentina back into third place.