For me personally Id be more apprehensive if we had Morata because I know so little about him other than what Ive read. At least with Lukaku I know what he's about, Ive seen it with my own eyes. Which gives me confidence he's going to be fine. Also the fact Mourinho believes in him and I believe in Mourinho.
Morata, I still have this nagging doubt based on the fact that his numbers just dont compare. Yes there are mitigating factors, but still, he has never scored 20 goals in a season and while you can extrapolate that he would if he played more regularly, that isnt the same as him actually doing it. Plus this whole "impact he has on the attackers around him" argument is so intangible. I can see the logic of it but not everything that sounds logical works. You can argue it the other way as well. I would say we have some pretty tidy, skillful attacking players, particularly Mata and Miki, also Rashford and Martial, Pogba and Herrera etc. We dont necessarily need anyone to lift their levels (I think they will lift their own levels, they dont need a striker to do it for them), we need someone to finish off the chances they make.
Im not saying that's right either, Im just saying you dont know how things will develop until you see it. So Ill reserve judgement until I see them playing, properly, in competitive games in the league. But I feel there are plenty of grounds for optimism.