The new arrival from city rivals 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 capitalised on a terrible error from Rossoblu goalkeeper Federico Agliardi to regain the lead for his side, after Diamanti had levelled with a spot-kick of his own in the first half.
And with the hosts scrambling to get themselves 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 visitors a much-needed goal after their opening-day defeat to Sampdoria last Sunday.
The hosts tried to gain a foothold in 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.
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 on 56 minutes when he produced a clever turn and snap shot that flew just wide of the post.
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.
He then found the side netting with a clever flick from yet another corner of his creation, in an impressive spell from the former Portsmouth man.
Marco Motta wasted a good opening for the Bologna after being played in by substitute Alberto Gilardino, who came on for Acquafresca. Milan responded by making a change of their 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 who 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 with 13 minutes remaining.
And Pazzini bagged his third on 85 minutes when he deftly flicked in Nocerino's low shot after the hosts had failed to clear their 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 home side, but after two defeats from two they will have to hope they can turn their spells of dominance into goals.