Inter have snapped up the midfielder in a Bosman deal.
"I am disappointed that Milan are losing Calhanoglu for free, but I am happy he is going to Inter, because he's an important player and a lovely guy," Mirabelli told Tuttosport.
“It's a great deal for the Nerazzurri, they will really enjoy themselves, but the Rossoneri will regret him."
Mirabelli also said: “I had to resolve many contractual problems at Milan, the problem is one of strategy.
“With (Gigio) Donnarumma and Calhanoglu, the club has lost €150m. If they're happy with that… The next problem will be (Franck) Kessie, these important renewals need to be sorted out earlier."
Donnarumma has committed to PSG this past week.