Confidence isn't any kind of guarantee of not making a hash of it. When Brucie arrived he immediately announced he was after Clemence's place. Super confident. Then he got the chance and spent much of that first season giving everyone terrifying frights with his bizarre decisions. So great that Karius is confident but I still wouldn't just throw him on in a big game like this.
Is it that big a game though, really?
If it was in April for a CL place then yes of course don't disrupt things, but it's so early in the season now that there's much less pressure on.
I think it'll come down to how firm he is on Karius being number one. If he bought him to start then nothing from Ming so far this season will have changed that view so he should go straight in.