People trying to defend a weak keeper by saying he's made some great saves is moronic. You go and watch a high school game and the keepers will make some great saves. They keep goal day in day out, that's what they are there for. You judge a keeper on his overall performance and like Carra said today, he hasn't shown anything that makes it look like we have much of an improvement, if any at all, on Mignolet.
With Mignolet in goal if the opposition hammers up a clearance our centre backs, or Henderson, have to chase it, facing goal. They then have to retrieve the ball and either pass it back to Mignolet or turn on it themselves. Mignolet then takes two or three touches on the ball before making either a simple pass to one of those defenders, who are now back around the edge of the box, and we then have to restart the attack from there, or he hoofs it long, in a team where the only player really capable of consistently making something of those kinds of balls is Origi. And that's not a criticism of Mignolet, by the way, who has been very good in just about every game he's played this season. Nor is it even a criticism of his distribution, which hasn't been bad at all. This would also be the same for just about any other 'keeper in the league and, of course, for Joe Hart, which is presumably why Guardiola shelled out £17 million for a 33 year-old who isn't actually that good a shotstopper (which presumably Guardiola had realised before having, you know, worked with him for years) - so we can safely assume that wasn't why he was bought.
With Karius in the team, the CBs don't have to chase it, because Karius has generally already come out to intercept it, and passed it, first time, to one of that back five, who are now restarting our attack facing goal and positioned around the half-way line. That's been obvious in just about every match Karius has played, including the first half today, and equally obvious in the match - against Leeds - that he didn't play.
Karius enables the whole team to push much further up for much longer and he recycles the ball far, far more quickly and with a lot more confidence than Mignolet does. If you haven't noticed the difference - if you can't see what Karius adds to the team, then it seems to me that there is a lot of the game you are not noticing. Positioning is important, and Karius improves ours, enables us to apply more pressure in attack and leave less space when defending.