Gabon international Aubameyang was linked with a return to Milan, the club he left for Saint-Etienne in 2011, but a transfer never materialised.
Instead, Milan signed Nikola Kalinic on loan from Fiorentina following the arrival of Portuguese forward Andre Silva.
"Aubameyang is a great player. He wanted to come to Milan, but some marriages just don't happen, for one reason or another," said Mirabelli via a livestreamed interview.
"Borussia Dortmund wouldn't sell and that's the way it went, but he remains a great striker."
It was a busy transfer window for Milan, who overhauled their squad with the signings of Leonardo Bonucci, Lucas Biglia, Hakan Calhanoglu, Ricardo Rodriguez, Mateo Musacchio, Andrea Conti, Franck Kessie and Fabio Borini.
And Milan have made a strong start to the season, winning their opening two Serie A fixtures after qualifying for the group stage of the Europa League.
Mirabelli added: "I read people say we are missing a winger and a wide midfielder, but we hope that those are the only two things we got wrong! We think we've done a great deal, because 11 buys are not easy.
"The team needed to be assembled, so it was impossible to fill it with too many new faces. In any case, January isn't far away, and we are ready…
"The intention is to open up a new era, so it's crucial to keep the average age of the squad right. We have medium to long-term objectives, but naturally we cannot think of repeating this transfer campaign.
"We realised that we had to get in there early, thinking some might underestimate us."