No one knew before Donnarumma proved it, but one thing is certain and must be acknowledged: Donnarumma is a great goalkeeper, and he has proven it, especially since his young age allows him to quickly adapt to the playing style implemented by Pep Guardiola.
I find Manchester City's early season experience a bit confusing. Despite having a strong squad across all aspects and a great coach like Pep, their performance hasn't improved. So, there are things beyond technical aspects that Pep needs to improve, perhaps tactics or player chemistry.
You made great points about Donnarumma and Manchester City. You are right that Donnarumma is top goalkeeper and his youth and ability to adapt quickly would make him perfect fit for coach Pep Guardiola. When it comes to Manchester City it is frustrating to see them struggle despite having great team and world class coach. Problem likely is not just about their skills. I think their can be low team chemistry or issues with their tactics. Pep will have to figure out what is wrong to get them back on track.
Yes, we all agree that Donarumma is indeed a very great goalkeeper, and this was indeed proven when he was still a PSG player. But I think that is still speculation at this time. Because until Donarumma plays on the field, we certainly won't know how he will perform. Because he could play very well at PSG, and play poorly now at Manchester City. Although I personally also hope he will be the solution to Manchester City's current weak defense. Because, as we know, Manchester City's defense and attack are currently quite weak and seem less than optimal. Because in the last few matches, Manchester City has really had very poor results.
So with that, hopefully, Donarumma's arrival at Manchester City can at least cover some of the gaps that Manchester City has. Furthermore, speaking about the cause of Manchester City's current poor performance, I think the Manchester City players seem less motivated at this time. Perhaps this is because they often win the Premier League, so that this has prevented them from developing their skills properly. So, with that in mind, I think Pep needs to be able to rekindle his players' enthusiasm. Honestly, it's quite worrying to see a team as great as Manchester City so weak.