Sirigu, 32, had long been in the shadow of Italian goalkeeping legend Gianluigi Buffon, who retired after the four-time world champions' failed to qualify for the 2018 World Cup in Russia.
But just when it looked like the Torino goalkeeper was the most obvious choices to fill the void left by Buffon's retirement, AC Milan's 20-year-old stopper Donnarumma stepped in to fill the large void left by former captain and record appearance maker Buffon.
"Sirigu knows what the pecking order is, we know that we can count on him and on the others," insists Mancini.
"Donnarumma is the first-choice, but in that position we are very well covered."
Donnarumma, who has 14 caps, is coach Mancini's first choice ahead of Sirigu, who has earned 22 caps going back to 2010.
Mancini took over after Italy's World Cup qualifying flop and is rejuvenating the national team, with the youthful Donnarumma and others such as Inter Milan midfielder Stefano Sensi seen as a investment for the future.
After becoming the youngest ever goalkeeper to play in Serie A at 16 years and 142 days of age in 2015, Donnarumma did the same for his country the following year, earning his first Italy cap aged just 17 years and 189 days.
The pair have played three matches each during the first six Euro 2020 qualifying games, with Sirigu conceding just one and Milan's Donnarumma letting in two.
Milan have had a poor start to the season with four defeats in seven games, and Donnarumma has conceded eight goals in the six matches he has played, while Sardinian Sirigu - who won four Ligue 1 titles with Paris Saint-Germain - has conceded 10 times as Torino sit ninth in Serie A.
Donnarumma pulled out of the weekend's game over Genoa, which Milan won 2-1, with sickness but Mancini said that Donnarumma would be fit to play against Greece in Rome and at Liechtenstein on Tuesday.
Italy sit top Group J with a 100 percent record from their six matches, and they can seal their spot in Euro 2020 with a win against the Greeks.
Regardless, Italy are almost guaranteed a place in the Euros as they are nine points ahead of third-placed Armenia with four matches remaining.
If they fail this time, they will have a second chance when they visit Liechtenstein on October 15.
Former Italy international Mancini has thrown his trust behind young talent such as Donnarumma, Inter Milan midfielder Nicolo Barella and highly-rated Roma attacker Nicolo Zaniolo.
In keeping with the team's theme of renaissance the 'Azzurri' will line out with their new green kit for the first time against Greece this weekend, only the second time they have worn the colour.
"I'm old fashioned, I prefer blue or white," insisted ex-Inter Milan and Manchester City boss Mancini, 54.
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