Milan appeared to be heading for their first league win in more than a month on Saturday following goals from Olivier Giroud and Tijjani Reijnders but Lecce drew level in the second half with two goals in four minutes from Nicola Sansone and Lameck Banda.
Lecce had what would have been a stoppage-time winner ruled out by the video assistant referee, moments after Milan forward Giroud was sent off.
Milan were hoping to get back on track in Serie A after ending a winless run with a 2-1 victory over Paris Saint-Germain in the Champions League on Tuesday.
The Rossoneri were already without a number of players, including Christian Pulisic, and their injury crisis deepened when Leão limped off in the 10th minute.
His replacement Noah Okafor had a hand in the opening goal in the 28th minute as he played a one-two with Theo Hernández, who crossed for Giroud to chest in.
Milan doubled their lead seven minutes later with a superb solo goal from Reijnders, who raced downfield from near halfway before firing past Wladimiro Falcone.
Milan had let a two-goal, halftime lead slip in a 2-2 draw at Napoli at the end of October and the same was to happen at Lecce.
Blin nodded on a corner three minutes later for Sansone to tap in and the latter was also involved in the equaliser as he touched on Piccoli's cross for Banda to fire into the far bottom corner.
Sansone almost scored the winner five minutes from time but his header came off the left post and then rolled across the face of the empty goal.
Giroud was sent off in stoppage time, receiving two yellow cards in a matter of seconds for continuous dissent.
Piccoli then appeared to have scored with a long-range strike but home fans' joy turned to dismay when it was ruled out for a foul by the young forward.
Two dead-ball situations for Filip Kostić. Two goals for Juventus.
After a drab first half, Juventus broke the deadlock against Cagliari on the hour mark when Kostić whipped in a free kick to the back post where Bremer was left unmarked to head into the far side of the net.
Juventus doubled their lead 10 minutes later. Kostić's corner came off Daniel Rugani's chest and onto the crossbar but Rugani chested in the rebound.
Cagliari's goal also came from a corner and was headed in by Alberto Dossena.
It was the first goal Juventus had conceded in more than 600 minutes, since a 4-2 loss at Sassuolo in September.