Atalanta surpassed Milan, their regional rivals, 2-1 in Bergamo to take over first place in Serie A, their ninth straight league victory.
With an early goal from De Ketelaere against his former club and a late winner from Lookman, La Dea beat another Italian heavyweight, following last month’s triumph over Napoli.
Just moments after falling behind, Morata had Milan level, and it appeared like the Rossoneri would be satisfied with a draw.
However, Atalanta scored their third goal in as many matches, and Milan’s defensive efforts were foiled at a set piece.
As a result of being more clinical in the last third, Gasperini’s squad ended with an xG of 1.59, while the visitors’ had a more modest 0.62.
But they can’t relax for long: next Tuesday, Real Madrid will visit the Gewiss Stadium for a Champions League match, and the following week, Milan will face Red Star Belgrade.
Atalanta attacker and Super Eagles player Ademola Lookman was awarded the game’s most valuable player after the match.
The 27-year-old was instrumental for the home team throughout the match, appearing for all 89 minutes and scoring the game-winning goal after having five shots on goal and creating one.
