The 18-year-old goalkeeper told Milan last week he did not wish to renew his contract, which runs until the summer of 2018, opening up the prospect of a possible transfer this summer.
However, Mirabelli has insisted the Italy international won't be sold, despite the San Siro club running the risk of losing the highly coveted keeper for free next summer.
"Donnarumma is a great champion, a great guy and a Milan player," Mirabelli told Sportitalia. "For this next season, he will certainly play with us still.
"It's the coach who makes the decisions. They say who will make the bench or sit in the stands -- it is not in heaven or on earth.