He has to start on current form and even when we get the likes of Martial and Lingard fully fit (I doubt they'll be at the required level of fitness until after the international break), he'll probably still start assuming he keeps up a good goals to game ratio.
Somewhat surprisingly, he's our top scorer in all competitions with 15 goals in 37 appearances (11 apps as a sub) without taking penalties. For comparison, Rashford has 11 goals in 36 appearances (10 apps as a sub). Martial has 11 goals in 28 appearances (7 apps as a sub), and Pogba has 14 goals in 35 appearances (2 apps as a sub). A little bit off topic, but Lingard and Mata only have 5 goals a piece, we really should be expecting these guys to get into double figures for goals considering they will make well in excess of 30 appearances. They're getting bailed out a bit by Pogba hitting the numbers this term, but when he isn't doing it (as seen under Jose), we'll struggle to find the goals from our more 'no.10 / wide right forwards' to take the pressure off the main forwards.
I know we all complain about Romelu's touch, but in the box he is as dangerous as most of the world's top strikers. He needs a specific kind of service with quick cut-backs and crosses that he can run onto and just bury. He scores so many goals inside the box when the full backs or wingers run beyond to open the game up. If we can keep doing that he will hit 20+ goals this season quite comfortably. Rashford, Martial, and Pogba should all comfortably hit 15+, probably more like a combined total of 50 between the three. Then, again, Mata and Lingard will probably need to pop up with a few more each at some point in a game which could be the difference between success and failure in the end.
I've discounted Sanchez here as he's just not managed to get anywhere. While he has been very poor, injuries have definitely interrupted his momentum and ruined any chance he had to work his way into some form. However, if he can stay fit and adapt his game (he should be able to do this, he's obviously got the quality), then we shouldn't be accepting less than 15 goals next season from him as well.