As a striker, you're always stuck in the same attacking system though.
The rate of shots on target to goals has been consistently 3:1 (and a few fluctuating decimals) since Charles Reep took a miner's lamp, a notepad and a #2 pencil to a game.
So if the standard for a striker is 33% accuracy, that means the standard for keeper saves (because a shot on target can only be a goal or a save) must be 66%. And if that's the case, then we can surely look at keepers with the same standardized view, and judge a keeper to be "above average", "average" or "below average" at least at shot-stopping, which is their basic task, no?
This is the mean. As I said, it's more the frequency of shots that is dependent on the attacking system rather than shot accuracy itself, because shot accuracy is completely down to the player. Of course, he will face some buses, but since he's only relatively compared with other strikers who face the same opponents as himself, there's not a huge deviant because the standard of opponents in defense will come to a mean for all the strikers.
This is not quite the same for goalkeepers, because their effectiveness is more dependent on their own setup of the defense (considering a decent to large sample space) than anything else.
I would say why we cannot standardize the save percentage for GKs yet.
1) xG factor and beyond xG. From an attacker's point of view, the attacker actually decides how much better he is from the xG standards. So, the more shots the attacker gets on target for relatively less xG, the better his finishing is. But the GK, does not actually decide how much better he is to XGA. First, the attacker dictates the position of his shot. So, the xGA is already set even before the GK is called into action. Second, the way a team is setup decides the xGA they can expect to concede. This again is not decided by the GK. Beyond xG, xG also doesn't calculate the number of defensive players between the attacker and the GK. So, while calculating shot accuracy, this becomes a lesser factor because the striker faces all the opponents who have conceded xGA at various rates. However, this becomes an increasing factor for the GK, because the GK always stands for the same defense which has conceded more/less xGA and in turn looks better/worse than he is.
2) The impact of an attacking system on a striker while attempting to shoot on target, is much much lesser than the impact of a defensive system is for a GK who is about to make a save. For a striker, who is about to shoot on goal with a specific xG, it does not matter the positions of his team-mates or how they attack, whether they played pass and move or whether they played a long ball, or whether they played a low cross to get to that position. It only matters that the striker is able to get the shot on target from that position and preferably beyond the GK. However, for a GK to make a save, the impact of the defensive system of his team is very important, throughout the move and right at the point when the shot is taken and the save has to be made. If a CB is successfully able to thwart an attacker to his non-favorite foot, same ball position but different body position, a few seconds delaying the attacker from the shot etc. are all able to impact how well the GK is going to make the save. Teams that consistently and successfully manage to do the above will help their GKs considerably without even affecting the xGA numbers that they face.
This is evident from the fact that the Save Percentages of both Mignolet and Karius have both dropped significantly from their times at the previous clubs. We may all discuss about our GKs and how they've regressed or have our doubts on our GK coach Achterberg, but the impact of our style of play is indeed high. We hardly have numbers at the back when we defend. Even worse when we defend counters. A few seconds of delay is lost. A bit of forcing hurried shots by tight marking is lost. A bit of pressure to change the striker's body position/angle is lost. Sometimes difficult shots look easy to score against us, because of the time and space they have. I'm very sure that a GK playing in Burnley's or even Chelsea's setup is a lot more protected (however good the GK maybe) than he would be at ours (both save percentage and xGA wise). If we sign Oblak, we will of course improve, but I'm sure that his save percentage will drop at LFC than at Atletico. And it will not mean that he regressed during one transfer window.
