Top keepers have around 80 % saves succes rate. Szczesny had around 68-69 % this season, but tbf it was one of his worst. Last season he had 74.4 which gives him place in top 10 goalkeepers in the world. Still its much worse than Oblak who gets around 80 % each season.
Donnarumma on the other hand never reached more than 71 % save success rate, but he is improving by each year.:
2017/2018 - 68.42 %
2018/2019 - no data
2019/2020 - 70.63 %
2021/2021 - 70.77 %
Szczesny:
2017/2018 - 72.73 %
2018/2019 - no data
2019/2020 - 74.42 %
2021/2021 - 68.93 %
Oblak is on different level though:
2017/2018 - 83.21%
2018/2019 - 79.95 %
2019/2020 - 76.72 %
2021/2021 - 80.47 %
Handanovic:
2017/2018 - 75.61%
2018/2019 - no data
2019/2020 - 71.68 %
2021/2021 - 70.27 %
now the question is how much can Donnarumma improve and is Szczesny really getting worse or it was just one season. Anyway this stats just shows how much better Oblak really is, only Courtois can be compared to him in last seasons and Navas maybe.
Courtois:
2017/2018 - 69.09 %
2018/2019 - 65.71 %
2019/2020 - 79.17 %
2021/2021 - 76.27 %
Navas:
2017/2018 - 72.81 %
2018/2019 - 74.19%
2019/2020 - 73.13 %
2021/2021 - 80.43 %
And Milan is trading Donna, for this:
Maignan:
2017/2018 - 67.68 %
2018/2019 - 74.02 %
2019/2020 - 68.60 %
2021/2021 - 79.09 %
Btw this are just league stats.