Don't think he made a mistake for Lacazette's goal - in hindsight he could've done it better yes, but that's just a very good goal by a very good striker, if he doesn't do that no one would bat an eyelid about coming for the ball in that situation. He was a bit slow to react for Belgrade's second goal but hardly a shocker, and yes the one were he was dispossessed was very poor but it didn't cost us and he seems to have learned from it.
I think he's already a good goalkeeper and has the potential to grow into a great one, one of the best in the league (hardly an accomplishment at the moment though). I think he'll be a bit more settled as the season goes on and in his second season and so on. Let's not forget that DDG didn't look like the best in the world either when he arrived in England, although granted he was 5 years younger than Alisson currently is.
Besides, how much football do some on here even watch? I doubt anyone watches all of United, City, Liverpool, Bayern, Atlético and Barça regularly enough to form an opinion on who is currently the best in the world.