Losing to Fiorentina would be acceptable if they didn't have dominance in the match, but AC Milan actually had dominance that made them look like losers. Look at the statistics, they had the advantage in terms of attack and possession, but they couldn't win against Fiorentina.
Dominating the match doesn't guarantee the win. There are many other factors that can make a team to win or to lose. In the match Fiorentina vs AC Milan, the goalkeeper becomes one of the factor of the win. De Gea performed very impressive, he was truly on fire in the match. He saved 2 penalties, he foiled several shots on target of AC Milan players. This makes AC Milan players to feel frustrated, they have no idea to score more goals. Meanwhile for Fiorentina players, this brings positive impacts on their mentality. They were getting motivated to win the match because they are confident with their defense.
I think we can't expect more from AC Milan this season.
If you expect AC Milan to win the Scuddetto, it is unlikely to happen. Although it is still to early to say this, but I'm sure the competition for Serie A title is for Napoli, Juventus, and Inter Milan.