Milan coach Sinisa Mihajlovic surprisingly chose Donnarumma ahead of the experienced Diego Lopez and his side went on to beat Sassuolo 2-1.
"I don't look at the age but only if he is good or not," Mihajlovic said. "It shouldn't be a drama."
Donnarumma was beaten by a free-kick by Domenico Berardi that made it 1-1 in the 53rd minute, but Luiz Adriano scored late to give Milan the win.
The youngest goalkeeper to play for Milan in the league was Giuseppe Sacchi, who came off the bench at the age of 16 years, seven months and 24 days in a 3-1 loss to Fiorentina - 73 years ago to the day.
It was his only official match for Milan.
Gianluca Pacchiarotti holds the record for the youngest ever goalkeeper in Serie A after playing the final 10 minutes for Pescara in a 1-0 loss at Perugia on 9 March, 1980. He was 16 years, 6 months, 16 days old.
Italy and Juventus goalkeeping legend Gianluigi Buffon was 17 when he made his debut for Parma.
