McVay hasn't been testing Mannion. At all. He is fine with Mannion. He is fine with the offense. The only thing they've been looking at is the running game. But he hasn't asked Mannion to even run the offense or given him any of the starting receivers to throw to. If he had any doubts or questions about Mannion, he'd be putting him in more game-like situations and asking him to make plays within the real offense.
I'm not a Mannion fan but I still wonder what it is that the fans have seen that they have such a problem with? Mannion hasn't been missing wide open guys or making terrible decisions. He's been making catchable throws to receivers who get no separation and who can't make contested catches. Statistics mean nothing in the preseason so I don't care about those. Mannion has looked fine with his throws and decisions. Not great or spectacular but I'm not going to pass judgment on him until I see him in the real offense with some real players around him.
As I said, that McVay is not asking him to do much of anything in the preseason, to me, means his job is safe for now.