There are some issues to consider. If you have very low shots against , that means the defense is great and those shots have very much lower chance to go in since the striker is under pressure, lower time to react, less time to position the goal attempt and many stressed factors. When you have many shots on target that means the defense leave great scoring positions to opposition, striker is under low pressure, the danger on each shot rises, more time to adjust and so on, less quality needed to score and you get more chances with higher and higher probability to score. There is a very clear correlation with lower chances offered and league position. Simple fact we relent so many shots on target against us means we suck at defense & mid and offer opposition much leeway to up the danger vs us. This requires shot stopping, not much else to it.
Problem is not with Onana, he is a known quantity, great with feet less so hands. Problem is with the club's over inflated self image to think we play in a way that is compatible with such a keeper. (It's not... yet). We are on path to a 140-180 shots on target against and if he can't maintain a 70+ save% then were looking at 8th or so finish.... I blame recruitment. Great keeper, wrong fit with us. Sure some argue we dont have the main back line, but arguably this IS the main backline since the "main" is more injured then not reality dictates this IS main.