If a player has a bad game or two in preseason, we are told to ignore it, because it's just for fitness etc. So I don't think a player who plays well should necessarily find themselves in the first team.
To be fair to Valery, he has played well before at CB a couple of times, but he isn't a CB. It was just a matter of time before he got found out. He isn't particularly strong, he's not a great header of the ball, and his reading of the game is quite slow. He's just not a CB. He's barely even a RB.