Dare I say reminds me a little of Sterling when I was watching him as a 16/17 year old kid in our u21s just before being called to the senior squad.
A fearless kid who integrated with the first team and didn't look out of place.
I agree, I think you can usually tell pretty quickly when a player is too good for the reserves, it's too easy for them. But often that doesn't mean they are ready for the first team, the step up shocks them because its not easy anymore and they go from being easily the best player on the pitch to struggling to keep up.
Doak looks like he's mentally ready, he's still pretty willowy at 17 but he's tough. He believes in himself and he wants to prove it.