The new arrival from city rival Inter was in fine form, and went some way to silencing his doubters with an opportunistic treble that put the Rossoblu to the sword.
The controversial new arrival put the visitors ahead from the spot, and capitalized on a terrible error from Rossoblu goalkeeper Federico Agliardi to regain the lead for his side after Diamanti had leveled with a spot kick of his own in the first half.
And with the host scrambling to get itself back in the game late on, he deflected in Antonio Nocerino's shot from 12 yards to wrap up a win that was far less comfortable than the scoreline suggests.
It was the Rossoneri who started brightest, and they were ahead when Nicolo Cherubin conceded a controversial penalty. After being caught out by a chipped ball over the top from the impressive Kevin-Prince Boateng, the defender dragged down Pazzini, who made the most of his fall. Cherubin was a little unlucky to have the decision go against him, let alone to receive a yellow card as well.
Either way, Pazzini's firm penalty gave Agliardi no chance and handed the visitor a much-needed goal after its opening-day defeat by Sampdoria last Sunday.
The host tried to gain a foothold on the match and Taider did well to create space for himself after 31 minutes, but miscued his shot and fired well over the bar with his left foot. It was a good opportunity for the host, which hitherto had only threatened through Diamanti's driving runs.
The Rossoneri's deadline-day signing Nigel De Jong was introduced to the action somewhat earlier than he would have expected when Riccardo Montolivo went off with a hamstring injury four minutes before the break, but within seconds his new side had conceded a penalty.
It was Robert Acquafresca who earned it when he nipped in front of Nocerino and was caught by the Italy international on the corner of the area. Diamanti - Bologna's best player by far - stepped up and sent Christian Abbiati the wrong way to draw his side level.
The Rossoblu flew out of the blocks after half time and they almost went ahead when Tiberio Guarente fired wide from Acquafresca's cut-back.
Boateng continued to provide Milan's biggest threat and showcased his talents brilliantly after 56 minutes when he produced a clever turn and snap shot that flew just wide of the post.
And the Ghanaian was prominent on the hour mark when he first slipped in Nocerino with a sublime first-time pass that eventually earned his side a corner, before then making space for himself on the right-hand side of the area and forcing a smart save from Agliardi, and then finding the side netting with a clever flick from yet another corner of his creation.
Marco Motta wasted a good opening for the host after being played in by substitute Alberto Gilardino, who came on for Acquafresca. Milan responded by making a change of its own, and threw on former Barcelona forward Bojan, who arrived on loan from Roma late in the transfer window.
Fellow debutant De Jong was booked for unceremoniously bringing Archimede Morleo's forward surge to an end, and Gilardino should have done better when he headed Diamanti's resulting free kick over the crossbar.
But it was Milan which got the third goal thanks to a horrendous error from Agliardi. Antonini's cross from the left looped up in the air off Motta and was fumbled by the home goalkeeper, allowing Pazzini to fire home the loose ball from close range after 77 minutes.
And Pazzini bagged his third with five minutes on the clock when he deftly flicked in Nocerino's low shot after the host had failed to clear its lines.
The former Inter man may not have been a popular arrival when he joined from the Nerazzurri in part exchange for Antonio Cassano, but he put in a heroic performance here to earn his side a much-needed victory. There were encouraging signs for the host, but after two defeats from two it will have to hope it can turn its spells of dominance into goals.
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