Zlatan, while more prolific in his thirties, was definitely a better player in his late 20s imo, all around class act, seeing is believing. I do feel that Lukaku suits our system better even if he is a similar level, perhaps slightly worse, perhaps slightly better, in comparison to Zlatan last season. I do feel as though someone like Lukaku, with his mentality, work ethic and avid watching of the sport, will continue to improve and I would say that we are probably 3-4 years away from his peak seasons. In this season alone I have noticed that Lukaku's passing and hold up play has improved significantly, as well as his vision, he just needs to be a bit less clumsy on some occasions and finish a few more of the sitters he has missed. Overall I'm very happy we got Rom, in retrospect he was definitely the better player to get over Morata, hes physically stronger, but also much stronger mentally, a student of the game. (He also seems to have a wicked cross on him.)
I split Zlatan last season into two groups though, Zlatan first half of the season vs Zlatan second half of the season (after christmas), imo his performances declined significantly after about early January. Lukaku this season we will see but I can see him kicking on from here and having his best run of the season aside from the first ten games at least in the rest of the season.
Overall so far I still think Lukaku this season, this is reflected by the teams overall output, all the attacking players around him have been much more prolific, and I don't think that is a coincidence, I mean of course players improve but no doubt Lukaku being there instead of Ibra certainly changes up the dynamic of the team going forward. You can't discount Matic's impact either however.