The presence of De Gea in Fiorentina does have a big influence on the match, because if Fiorentina does not play De Gea in the match I am sure Fiorentina will lose. Because anyway, based on the head to head results, at least Milan beat Fiorentina quite often but at that moment Milan failed despite dominating the match well. Fiorentina managed to make Milan get a deadlock, despite having two penalty kicks but De Gea managed to handle it very well.
Indeed. De Gea contributed a lot on that match. He deserves to be man of the match. It was just incredible that he saved 2 penalties and blocked some shots on target. If we learned the squad quality and historical aspect of head to head, surely AC Milan is superior. But De Gea surprised AC Milan, he performed like the tough wall. 
AC Milan lacks a goal-hungry striker, because they managed to dominate the game when they faced Fiorentina but unfortunately many of the attacks they built were wasted because they could not be completed into goals. 16 shots and 8 of them on target of course that's a pretty good record but in fact they failed to make the opportunities available to become goals.
I think it is untrue if AC Milan has no sharp striker. AC Milan has 2 strong strikers, Tammy Abraham and Álvaro Morata. If they wasted many chances, it was because the quality of the opponent goalkeeper. IMO, AC Milan has done their best but they were unlucky that De Gea performed brilliantly.The defeat from Fiorentina made AC Milan's position in the standings drop to 6th place. I think under Fonseca's direction where AC Milan has not found its best form because they also failed to advance well in the Champions League this season. Of course this season is a tough challenge for Fonseca in building AC Milan but we'll see because this season the journey is still quite long.
AC Milan problem is about inconsistency. AC Milan could beat Inter Milan but they were defeated by Fiorentina. Fonseca needs to keep the good mentality of the whole players. He also should consider to change the tactics if it is needed. If he always used the same tactics and same lineup, the opponents can be easy to learn and break it.