Pato is great, but he is NOT best striker in the league, nor is he top 5 in the world ( not at all). He has the skill to become that, but he is too erratic yet, as is expected due to that age. For example he was a dud 2nd half of last season, overally Zarate was better then him, and he isnt better then Zarate in overall, nor more talented. Just another "random" name that is better then him too IMO is Aguero. Defenitely needs to do more if he is to be this exaggerated.
But Milan would be in real trouble if he was injured for a long time, not because he scored 2 goals today, but because their whole attack, after Kaka is gone, seems to be (besides hoping for R80 great assists, which he did today) of that Pato is supposed to run and dribbler the opposition defence on his own. It can get rather quickly for them if they dont have this approach.