Thing is we play him solely to score goals and play with two holding midfielders to protect the defence. So if we wanted him to cover ground all day then we would have a more industrious striker.
Not all strikers are the same and van nistelroy and Ibrahimovic for example don't run the channels or be some all round striker. They score goals and win games, and lukaku does that in spades in this team. We would have been down well before now if we didn't have his goals, and almost got to the final of the fa cup, only that it was one game too many for him to carry the team on his shoulders