Donnarumma's agent Mino Raiola has made no secret of his desire to take the teenage goalkeeper away from San Siro, although Juventus have rejected speculation they are interested in the Italy international, long considered an heir to namesake Gianluigi Buffon, who is expected to retire.
A pair of high-profile errors in the Coppa Italia final dented Donnarumma's reputation, but a deal to sign Reina from Napoli on a free transfer was in the works prior to the Rossoneri's crushing 4-0 loss to Juve in Rome.
And Mirabelli says the 19-year-old's future will not be dictated by the signing of Reina, which is yet to be officially announced by either Milan or Napoli.
"We had the opportunity to get him for free - and we think it was useful strategically," Mirabelli told Mediaset Premium of Reina.
"We'll have a lot of competition, Reina can help us off the pitch, in addition to his contribution on it. We need people with experience and he's one of those.
"I already told your other colleagues [in the media] that it's not just down to Donnarumma, it involves everyone. A player has to come to us and tell us he doesn't want to stay. That's not the case with Donnarumma.
"If that does happen, there would also have to be a club ready to meet Milan's asking price. For now, neither condition has been reached."