Midfielder Sanches, replacing Andre Gomes, breaks the previous record held by Cristiano Ronaldo who made his first championship start as a 19-year-old in Euro 2004.
Benfica full back Eliseu also came in for the injured Raphael Guerreiro.
Captain Ronaldo could also equal Michel Platini's record of nine goals at European championships if he scores in Marseille.
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Poland coach Adam Nawalka named the same team that beat Switzerland on penalties in the last 16. Lukasz Fabianski continues in goal as Wojciech Szczesny is still struggling with a thigh injury.
Young central midfielder Bartosz Kapustka, who was suspended for the Switzerland match, is on the bench.
(Editing by Ed Osmond)
