Donnarumma might be PSG backup option if Chevalier experiences an unexpected injury or the goalkeeper, as PSG already has a new goalkeeper this season. However, we haven't seen his full potential at PSG, so I can't yet judge whether Chevalier is better than Donnarumma. Let's see how he performs before PSG actually uses him as their first-choice goalkeeper.
Against Angers, they didn't do much, so PSG need to play a tougher team first to see how strong he is in guarding PSG defense.
Donnarumma is one of the best goalkeepers in the world and might not accept being a reserve player. It is in the public domain that the Italian wants to leave the French club. But no deal has been secured because of the price PSG placed on him. They were asking for as high as €40m-50m, which might have scared clubs like Manchester United, Chelsea and Manchester City away.
But based on news from some news outlets, the club has reduced its price to €20-30 million. PSG might risk losing for free next season if he remains in France.