He'll probably hinder Rashfords development which is a shame. Lukaku's confident because he's consistently near the top of the scoring charts which is (or should be) the primary rating factor for a striker. His allround play has actually improved substantially under Koeman, but everyone focusses on those "Lukaku hilarious first touch" videos on youtube instead of watching the player play. His frame makes him look lazy and a bit awkward sometimes, yet he's way more technical than he looks.
Honostly you might be right and he may struggle with tighter defences, but he's never failed to deliver up until now (except a mediocre record for the international team which I blame on lackluster coaching/low quality staff and total lack of team cohesion). I wouldn't count on it