The substitute came on the devastating effect, firing in the Rossoneri's winner as his team maintained 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 upfront after the latter returned from suspension, with Antonio Nocerino and Marco Amelia also handed rare starts as Milan re-mounted its bid for Champions League qualification next season.
The home side kicked proceedings off knowing the implications the result would hold on its European ambitions, and within two minutes had what the club felt was a cast-iron penalty appeal 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 Ignazio Abate's tantalizing low cross, as the Rossoneri continued to up the pressure.
But in perfect defiance, it was Catania which 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 visiting side's 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 pile driver at full stretch.
Sharply stirred by the shock of conceding, Milan swarmed back at its opponent. Kevin Prince-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 halftime 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 the club's endeavor, the European wannabe was punished for another defensive lapse when a static rearguard failed to pick off a slide-rule pass for Gonzalo Bergessio, who siezed 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 finishes 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 was 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.