Malaga's perfect debut season in the competition may have been sullied by a first goal conceded - and the loss of their 100% winning record - but with Milan and third-placed Anderlecht due to play each other once more, it is impossible for both sides to dislodge Manuel Pellegrini's men from the top two.
Milan are in pole position to book second spot as Pato capitalised on fine work from Kevin Constant to register his first Milan goal since December last year, edging the Rossoneri one point clear of Anderlecht, who beat Zenit St Petersburg 1-0.
Both sides prodded and poked in the early exchanges, Eliseu failing to properly work Milan keeper Christian Abbiati with a tame shot, before Constant tested Willy Caballero at the other end.
But Milan soon turned up the heat, Bojan Krkic thundering into the box to force a fine save from Caballero and Stephan El Shaarawy seeing his dangerous effort deflected behind.
Urby Emanuelson had Caballero at full stretch with a wickedly swerving free-kick after Martin Demichelis' foul on Pato, as Massimiliano Allegri's men exerted their authority.
Box-to-box man Riccardo Montolivo confidently defused Malaga's intricate build-up play, but Nigel De Jong's less-cultured approach in the 33rd minute earned him the first booking of the evening.
The Rossoneri would pay dearly for a rare moment of ill-discipline, with Isco ensuring the Spanish side made the most of it.
The 20-year-old midfielder weaved through the Rossoneri backline to supply a perfectly-weighted ball for Eliseu, the Portuguese swerving Constant's challenge to beat Abbiati with his right foot.
Milan had been caught cold and Emanuelson's wayward drive over the crossbar was certainly not the clinical response the Italians required before the break.
The Dutchman did warm Caballero's palms with a more accurate strike as play resumed, courtesy of Bojan's cool pass, but El Shaarawy's shot on the turn left much to be desired.
In the 73rd minute, Pato wriggled free in the box to meet Constant's left-wing cross and head home his first goal of the season.