Oddly enough, last season Lukaku was more prolific against the bigger teams than the lower teams, one of the factors which contributed to this is the space he was given by the opposing team.
Since, against the bigger teams, he was allowed more space, with them needing to attack and win the game. Whilst against the smaller teams, with the teams being more defensively orientated and giving him more attention, his first touch was exposed and hence he went missing in a lot of these games.
However, if he can work on his first touch, there's no doubt he will be a top player, given that he's still only 21.