To be fair, unless we somehow landed Lewandowski or Aguero (which we had a chance of pretty much nill) almost any player would have been a downgrade. As a pure #9 even at his age with United Zlaten was truly magical. Just a ridiculous combo of being able to hold people off the ball to lay-it off, first touch/close control, finish in the air, finish in the box, etc. While not the best of his generation (which goes to guys like Messi and Ronaldo), his overall set of skills is among the most unique I've ever seen to have such ridiculous technique at 6'5.
I don't think any of the primary rumored targets that summer would have gotten close to being Ibra. If I remember correctly it was Morata, Lukaku, and Belotti (I could be missing a fourth). Not to say, I'm not really glad with how we re-couped most of what we spent on Lukaku, and have replaced him quite easily with the emergence of Martial, Rashford, and Greenwood.