The current Torino boss handed Donnarumma his professional debut in October 2015 at the tender age of 16, dropping the experienced Lopez to the bench.
Berlusconi was not happy with Mihajlovic's call and confronted him on the training ground, only for the latter to tell his boss to either fire him or accept his decision.
"Everybody was questioning my decision to play Donnarumma because we also had Diego Lopez and Gianluigi was only 16. But he is now part of the national team and is priceless. Giving him his debut will always be something I am proud about," Mihajlovic told Corriere della Sera.
"Did I take a risk by playing him? When I played Donnarumma, Berlusconi came to Milanello twice to tell me that I had to play Diego Lopez. I told him that he had two options. One option was to fire me and put Diego Lopez back in the team, the other was to keep me and see Donnarumma play. He decided to keep me, fortunately.
"My time at Milan was difficult right from the start. We had lost some valuable time early in the season because president Berlusconi wanted us to play in a 4-3-1-2 formation, even if it was clear that formation was wrong for us. I had had enough after the game against Napoli and decided that if I was going to lose my job, I might as well go out on my own conditions and playing philosophy. And then the results slowly started to come.
"It was an honour to get to know Berlusconi and I will always be grateful to him. He was the best president out there for 29 years of his 30 years at the club. The 30th year was when I was there..."
Mihajlovic eventually left Milan in April 2016 after an underwhelming 2015-2016 campaign.