Donnarumma has been criticised for his role in Paris Saint Germain's elimination from Europe's premier club competition.
The Italian international was dispossed by Karim Benzema leading to Real Madrid's equaliser in an eventual 3-1 defeat on Wednesday.
PSG looked to be in control of the tie until Donnarumma's error led to Real Madrid's revival and the French giants' capitulation.
"Mistakes like this will serve him; they are part of his growth path," Buffon told Corriere dello Sport. "I'm sure it won't have any negative repercussions, any problems for him.
"Besides, he's already shown he has the necessary strength to overcome delicate moments in his career. He's too solid to allow himself to be influenced by all this.
"He's one of the best in the world, among the top three along with Thibaut Courtois and Manuel Neuer. I have to say that otherwise I'm accused of being partisan!"
Taking aim at Donnarumma's critics, Buffon added: "Errors from goalkeepers are normal. Gigi can still give a lot for PSG. One or two mistakes a season are psychological.
"He made an unpopular choice joining PSG, but it must be respected. He wanted to fuel his ambition, something many didn't like.
"They're the people who were waiting for this mistake. The problem is not the French who are enjoying it, but the Italians who were waiting for that."
Donnarumma has kept eight clean sheets in 18 appearances for PSG this season.
