Eranio was with Milan when Aubameyang was playing for the club's primavera team.
He told TMW: "We thought he was ours, and he grew up, just as many players think they can not claim anything, and then they show their own worth.
"He grew up well, even thanks to the work done at Dortmund. I hope he can continue this way."
Eranio added: "Well, there are so many good strikers, if I have to name one, I think Arek Milik (of Napoli) is a player who is worth a gamble."