Give him time because he is young, but he HAS to work harder on the pitch imo. Whatever your level, you can always work hard.
If you're Messi, take a breather, have a stroll sometimes. Everyone else needs to work like a b@stard.
How much time? He wont see much more game time this season, barring early FA Cup rounds, and he doesn't really deserve too. He could at least run so that its clear that he's making an effort. Janssen hasn't been as good as we had hoped this season but no-one can doubt that he's working his nuts off. Onomah needs to go out on loan - hopefully someone can get him actually putting some effort in. He also looks like a kid playing in men's football at the moment, so needs to overcome that transition and the PL isn't the place to do that