Relax, he'll stay. He came on at the same time that Greenwood did today. Same as what Chong had in the last game, which indicates that while Greenwood is deservedly the apple of everyone's eye, Gomes and Chong are considered to be his peers by the management.
He'll have his opportunities in the Europa League and I'm sure he'll force his way in to the reckoning by Christmas if he carries on this form.
Atm he's guaranteed a regular spot on the bench at the very least simply because we haven't bought any midfielders in and his versatility of playing any attacking role (beside striker) and any midfield role, means he'll likely get much more minutes than otherwise.
Also, look at his social media activity. Does that look like the activity of a boy who's unhappy? He genuinely loves the club.
His slight frame also means that he needs to be patient - not to the extent that Lingard or even Scholes were, but definitely enough to be content with regular substitute appearances and occasional starts before he's fully filled out.