Genoa fans turned their backs to the pitch in protest during a 5-0 home defeat by Atalanta and Diego Falcinelli injured himself celebrating after his late goal at Chievo gave Crotone their first-ever Serie A away win.
Leaders Juventus were away to third-placed Napoli in the evening game with Gonzalo Higuain returning to the San Paolo stadium for the first time since his acrimonious split last year.
Milan gifted the Dolphins an early goal when Donnarumma missed the ball completely as he tried to control Gabriel Paletta's back pass and it flew straight into the goal.
Mario Pasalic levelled before halftime but, despite dominating the second half, Milan failed to find a winner and were left seventh with 54 points.
They dropped four behind of fifth-placed Atalanta who were sent on their way when defender Andrea Conti met Leonardo Spinazzola's cross with a superb overhead kick after 25 minutes.
Read the full article on eurosport.co.uk: Milan held after rare Donnarumma mistake