Fellaini's current deal with the 20-time English champions will expire at the end of next month, but it had been thought that the Belgium international would pen a fresh contract at Old Trafford.
The 30-year-old has been strongly linked with Milan over the last 24 hours, however, and now Mirabelli has insisted that the midfielder will be available this summer having decided against staying in Manchester.
"Fellaini could be an important player, he is. We know he is not going to renew with United, but it's difficult to say that we have closed the deal. He has important attributes," Mirabelli told Sky Sport Italia.
"We will do something in that role but we are keeping everything in reserve. First we have to secure our signings, then announce them."
Milan are believed to be prepared to hand Fellaini a three-year contract this summer.