Roma winger Jeremy Menez, deputising up front for the injured Francesco Totti, ran on to Thiago Silva's mis-hit back-pass to slot in the opener on three minutes.
Ronaldinho again offered little in open play but levelled from the penalty spot on 56 minutes after Nicolas Burdisso was adjudged to have fouled Alessandro Nesta despite Roma protests.
Brazilian Pato soon chested down a long ball and finished calmly to give Milan only their third Serie A win of the campaign before Wednesday's Champions League trip to Real Madrid, lifting them to eighth in the standings.
Milan's Massimo Ambrosini was harshly dismissed late on.
Earlier, surprise pacesetters Sampdoria failed to rejoin Inter Milan at the top after being denied a strong late penalty shout in a 1-1 draw at Lazio.
Luigi Del Neri's side took the lead in an open game thanks to Giampaolo Pazzini's powerful header on 40 minutes following a cute cross from Marco Padalino.
However, Lazio hit back quickly when Matuzalem burst into the box to score and the second half seemed to be fizzling out once Padalino and Lazio's Roberto Baronio were sent off.
Samp's Antonio Cassano then sparked the game back into life by striking the bar before the referee was unmoved by Pazzini being poleaxed in the area by Lazio goalkeeper Fernando Muslera.
"The only person not to see the penalty for Pazzini was the referee. What's serious is that he didn't even give a corner, confirming that the ball touched Pazzini and not Muslera," an angry Del Neri told reporters.
RECOVERED CONSCIOUSNESS
Inter, who crushed Genoa 5-0 on Saturday, hold a two-point lead over Sampdoria with Juventus and Fiorentina four points behind the champions in joint-third after Saturday's 1-1 draw.
Christian Maggio's late goal gave Napoli a 2-1 win over Bologna on Sunday in Walter Mazzarri's first game in charge.
Maggio converted a cross from Argentina striker Ezequiel Lavezzi, who angered Mazzarri by only arriving back from international duty on Saturday.
Colombian centre back Nelson Rivas hit his head after falling awkwardly in bottom side Livorno's 2-1 home defeat by Palermo. After being rushed to hospital, officials said he had recovered consciousness and was not in danger.
Udinese, who had to start with Serie A top scorer Antonio Di Natale on the bench because of a knock, lost 3-1 at home as Antonio Conte's Atalanta won their first game of the season.
In other results, Catania beat Cagliari 2-1, Bari won 2-1 at Chievo and high-flying Parma overcame Siena 1-0.
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