Aguero has made a blistering start to the new season and will become City's all-time leading goalscorer once he returns to fitness from a rib injury, but reports claim he could soon be on the move.
Milan have cash to spend following a recent takeover and were ready to launch a bid for Aguero in the summer, but decided it was not worth the effort after City missed out on Arsenal forward Alexis Sanchez.
The Italians were hopeful that City would sell if they landed Sanchez, but after the deal collapsed at the last minute, opted to keep their powder dry.
However City are expected to finally land Sanchez in the January window and that could be the signal for Milan to make their move for Aguero, who has constantly been linked with a move away from the Etihad Stadium.
Whether City would consider selling remains to be seen, but according to reports in the press, Milan are ready to chance their arm.