There's not a right/correct order Mike. Everybody is fighting for that position. If this week Lukaku is in a better condition/form than other 3, then he will start. If next week Lukaku is in a ******* horrendous form, he isn't in the group. I mean, what's so scandalous about that? This is a big club and club that fights for trophies week in week out, you can't promise Lukaku -- or anyone else for that matter -- anything. Whoever plays/trains better he will start next. Plus, let's not forget that Jose likes to play a certain type of a striker depending on what the game is. So again, there isn't a right order. I'm guessing Ba will get the least of game time next season though.
If Lukaku wants to be huge one day, he needs to learn to fight for his place. Seriously, everyone should.