Rapid decline for what was a highly rated South American youth prospect. The game is littered with examples like that though and of course the reverse of that too. He moved to Europe, didn't settle, then his development stalled as his game time dwindled and then he seems to have gone backwards, nobody saw that coming really, not even the armchair scouts.
Hasn't Gabriel Barbosa gone through something similar? Must be back on track if he's being linked with a move to Everton, so maybe there is a way back for Marlos. Not at City mind you but a decent level in Europe.