Roma lost 4-1 at Atalanta and missed the chance to move level on points with third-place Juventus. Roma remained fourth, a point above Napoli after Gennaro Gattuso’s side lost 2-0 at Lazio.
Milan was missing several top players, with a number of others joining star forward Zlatan Ibrahimović in the treatment room.
The average age of its starting lineup was again under 23 but it got off to the perfect start when Hakan Çalhanoğlu surged forward from kickoff and threaded the ball through to Leão, who fired it into the back of the net.
Statistics supplier Opta said the Portuguese youngster’s goal beat the previous record of eight seconds set by Piacenza’s Paolo Poggi against Fiorentina in 2001.
Milan later tweeted that the 21-year-old Leão’s goal was scored after precisely 6.2 seconds.
Leão thought he had turned provider in the ninth minute when he pulled back for Çalhanoğlu to turn in but it was ruled out because Saelemaekers was offside in the buildup.
Saelemaekers made up for that in the 26th as he tapped in after being set up by Theo Hernández following a rapid counterattack.
Milan seemed to be heading for a comfortable win before Domenico Berardi’s free kick was deflected in, a minute from time, to set up a nervy finale.