'We got distracted,' says Italian star after bizarre keeper blunder
Germany saw off Italy 5-4 on aggregate following a 3-3 second-leg draw to reach the semi-finals.
Joshua Kimmich, who scored the first goal for Germany, played a key role in the bizarre second.
Donnarumma left his goal after making a save to complain at the referee.
Kimmich, seeing the Italian keeper had drifted off his line, took a quickly-taken corner which was flicked into the empty net by Musiala.
"We got distracted. It can happen to concede goals like that. We'll work better on it next time," said Italy striker Kean who scored twice in the second half as Italy rallied from a 3-0 interval deficit.
Germany coach Julian Nagelsmann said the quick thinking was "world-class and unbelievably smart," and admitted "I didn't see it at all".
One person who did see it was 15-year-old ballboy Noel Urbaniak who noticed that Donnarumma wasn't paying attention and quickly threw the ball to Kimmich.
"He was very sharp, which isn't bad in a game like this, he did well," said Kimmich who autographed and presented a match ball to the teenager after the game.
"He'll get a free ticket to the next home game," promised German football federation Rudi Völler.
"I've never been a ballboy before, this is my first time, and it feels really good," Urbaniak told RTL.
Italy coach Luciano Spalletti blasted his team's lack of maturity over the goal.
"We saw that they take corners quickly and must not turn our backs," said Spalletti.