Fabinho didn't hit the wall.
All that stuff is a bit over played. Quality players are normally alright no matter what League they end up in: Sometimes managers, tactics or a million other things can have an impact of course. If Torreira had started poorly and finished well we would have instead heard the "it takes half a season to adapt to the (magnificent, incredible) Premier League" cliché.
I like Torreira and he's clearly a good player, but at least part of the reason he dropped off so badly was because he got a little bit found out by opponents and is a very poor athlete. He'll do better next season, but he's always going to be 5'5" and a level below even average Premier League footballers in terms of athleticism.
Ndombele would eat this League alive because he's of the highest quality and an oustanding athlete to boot. He plays 50 games a season no problem and he'd do that here just the same.