TAA is in great form, and he's definitely one of if not the best RB in the world.
He's certainly not generational though, he needs to stay hungry over 10-15 years. He has a very long career ahead of him.
It's very much unlikely he will become a Cafu because I doubt that Liverpool will be dominant for that long, I'd say he's world class but for how long.
Greenwood, let's see how he develops, he could be world class, he could be generational, but he could also just end up being a brilliant finisher like an Owen.
It's sad to say but we generally don't know what injuries will happen, if a player will stagnate, at 18 years old you can't see a players entire trajectory.
Mbappe for instance relies heavily on pace, when he gets to 25-26 he will have to slow down a bit and he won't be the same 'explosive' player, for that reason he would never reach the heights of Messi. It's why I never understood when France beat Argentina how the media and pundits were saying 'the changing of the guard, the next great' with them shaking hands, when it was far from the truth.
We would love to say that Greenwood is going to be the next big thing, and I as a United fan would love for that to happen, but let's judge that in 3-4 years after he's had a few seasons under his belt.