Missed this post. Drogba comparison is a pretty good one, although he was more established at the moment of transfer as he was voted Franch player of the year by fellow professionals. But, still, that was his first great season in terms of top football and at 26 years of age as you have already said. Also, he was, as most here would like to say "no PL proven" with many questions whether he will be a success at Chelsea.
Also, when you look at his stats in Chelsea, you can see that he wasn't so prolific in terms of goals but he is undoubtedly one of the best strikers of PL. His ability to score in biggest games and also his ability to stretch other defences and allow other teammates to score more goals was vital for Chelsea success.
Something that
@el3mel already mentioned when he said that key is not for Morata to reach Zlatan numbers but for others to lift their numbers because of the way he plays.