I just don't see Lukaku providing/producing more than Costa. Costa has done the business for multiple teams in multiple leagues, and has terrorized some of the best defenders in the game, who have also got the better of him in return at times (looking at you Kos/Mustafi 🙂 ).
Lukaku is solid but he's not proven at the top tier, and has done decent against big clubs but not more. He'll struggle not being the big fish in a small Everton pond for at least a few months next season which is too long for the champions to overcome.
