2 goals against QPR doesn't mean Jelavic is 'in form'.
A striker who scores two goals in a game has done his job in that game, and Lukaku needs a rest / kick up the arse. I'm not going to argue with you about the definition of form, because you have made your mind up about Jelavic and nothing will change that.
The fact remains, we can run Lukaku into the ground, or give Jelavic some time when his confidence is up as a result of this performance. He could be gone in a week and then your choices are Kone in a wheelchair, or Velios. I say use Jelavic while he's here. If he sucks balls against Norwich, sub him at halftime.