Cillessen had been a back-up to Tim Krul in recent games.
They both played in a memorable 2014 World Cup quarterfinal match, when Cillessen was taken off late in extra time of a 0-0 draw with Costa Rica so that penalty specialist Krul could take part in the shootout. Krul saved two kicks and the Dutch advanced to the semifinals.
The Netherlands will play all three games in Group C at home in Amsterdam, first against Ukraine on June 13 then against Austria and North Macedonia.
The Dutch team’s first warm-up game is on Wednesday against Scotland, which reported its first COVID-19 positive on Tuesday at a training camp in Spain.
The Scottish federation said midfielder John Fleck is self-isolating and will not travel to Portugal for the game.
“The rest of the squad was retested for COVID-19 and returned negative results,” the Scottish Football Association said.
UEFA allowed the 24 national teams at Euro 2020 to have three extra players in a 26-man squad to help cover for COVID-19 cases and mandatory isolation or quarantine.