They suggest he had a great season.
They don't tell you why.
You are equating the statistical performance with ability.
I'm saying that's not a smart assumption to make.
If he had outperformed expected goals over the course of a number of seasons it would be safe to assume he is a great goalkeeper. That's the case with De Gea.
For the vast majority of goalkeepers the expected goals stat will regress towards the mean, so unless you're absolutely certain (based on something other than xG) that Alisson is brilliant you should have your money on him being much closer to average next season.
The genesis for statistics like this probably came in baseball - they've been using them for a long time now. And they recognise that stats like this are more useful for determining what players are unlikely to repeat their performance.
What they are trying to do with xG is find a stat that reflects how much a goalkeeper impacted his team. They do that by judging a keeper by the saves he made compared to what the average goalkeeper would have done with similar shots. But the stat is far from perfect, because it can't control for all variables - it records two things, where a shot was taken from and whether it was saved or not. We all know there are plenty of other factors that will impact whether or not a shot goes in.
If you've watched him a lot and you think he's great, fair enough. I haven't, i saw him twice in probably the worst two games he had all season. They are probably about as a reliable an indicator of how he'll be next season as xG is.
