Ok let's play someone who can't score
While the whole scoring thing is obviously a big part of being a striker, I think Lukaku does two other things that make him way more valuable than Jela... he's a real target and he can actually hold the ball. Lukaku's in many ways a better Fellaini in that regard -- Fellaini was definitely better at pulling balls out of the air, but Lukaku's not too bad at it himself and once he gets it, he can wait for Mirallas/Barkley/Baines/Coleman to move a bit and find them (Fellaini wasn't particularly good at setting others up, IIRC). If (more realistically, when) we have to replace Lukaku, those would be the characteristics I'd look for.