Will Lukaku improve us? Given the release of Zlatan this summer it was obvious we were going to be in the market for a replacement. Only few strikers in the world can directly replace Zlatan in terms of leading the line, scoring goal after goal and being a major factor to get three points.
Given Zlatan is/was arguably amongst the top five strikers in the world it's difficult to say Lukaku will 'improve' our squad as such but easier to say he will maintain the standard of which we had last season.
Who would improve us? I'd consider Lukaku a like for like to Zlatan in a very loose term. A powerful player that has that presence in the box who look as if they are built for the Premier League. I can not see him adding anything different to our game than what Zlatan did. In my opinion the only players that would improve us after losing Zlatan are Suarez, Lewandowski and Kane. These three players that are a class above yet somewhat unattainable, not that we would want Suarez anyway, obviously.
So, who do we need? What we are missing and what we have been lacking all season are the creative players that play behind the striker. Obviously Fabinho would be a great choice, not because he is a creative player but because he would give Pogba the ability to play a more advanced role and really give him the freedom he needs to play his best football. Herrera could also then play a more creative role rather than a predominantly defensive performance.
To summarize, I'd personally consider this to be a poor transfer window if we do not sign the players to support the system which Mourinho wants to play and give Lukaku the creativity he needs behind him, which Zlatan didn't have last season. Unless we sign a Fabinho type player to help Pogba or an improved creative player that will provide more assists and movement we should just expect the same football we had last season.
By no means am I saying we won't sign a midfielder this window but just trying to put across that at the moment we are no better than we were last season with even with Lukaku (barring defense with Lindelof). Lukaku has all the potential for the future but that is another topic, I am more focusing on next season rather than five seasons down the line.