From what little we've seen, Onomah looks like he'd be better in Dembele's deep-lying role than in the CAM position. But we already have Dembele, Bentaleb and Mason ahead of him. Ergo, I'd agree with
@NaijaSpurs that a loan would probably be best for him to get consistent game-time and develop.
Other than that,
@Robspur12's point is also a good one - the backups to the Lamela/Alli/Eriksen trio aren't entirely convincing. But, that's a lesser problem, since at least they do exist, which wasn't the case for Dier and Kane. If we can grab a top-class CAM to provide some fierce competition for those spots (or even take Alli's position in that trio, which allows Alli to be moved back into Dembele's role), great - if not, eh, it isn't critical to our prospects this season.