The 20-year-old spent the season on loan at Swansea City from Bayern Munich, but failed to prevent the Welsh club being relegated from the Premier League.
Sanches was named Young Player of the Tournament at Euro 2016 as the Iberian nation triumphed in France but has seen his career stall since departing Benfica two years ago.
Santos will cut his 35-man squad down to 23 in the coming days as the European champions aim to emulate neighbours Spain in 2010 by winning the subsequent World Cup.
Portugal and Spain have been drawn together in Group B, along with Morocco and Iran.
Goalkeepers: Anthony Lopes (Lyon), Beto (Goztepe), Rui Patricio (Sporting Lisbon)
Defenders: Antunes (Getafe), Bruno Alves (Rangers), Cedric Soares (Southampton), Joao Cancelo (Inter Milan), Jose Fonte (Dalian Yifang), Luis Neto (Fenerbahce), Mario Rui (Napoli), Nelson Semedo (Barcelona), Pepe (Besiktas), Raphael Guerreiro (Borussia Dortmund), Ricardo Pereira (Porto), Rolando (Marseille), Ruben Dias (Benfica)
Midfielders: Adrien Silva (Leicester City), Andre Gomes (Barcelona), Bruno Fernandes (Sporting Lisbon), Joao Mario (West Ham), Joao Moutinho (Monaco), Manuel Fernandes (Lokomotiv Moscow), Ruben Neves (Wolverhampton Wanderers), Sergio Oliveira (Porto), William Carvalho (Sporting Lisbon)
Forwards: Andre Silva (AC Milan), Bernardo Silva (Manchester City), Cristiano Ronaldo (Real Madrid), Eder (Lokomotiv Moscow), Gelson Martins (Sporting Lisbon), Goncalo Guedes (Valencia), Nani (Lazio), Paulinho (Braga), Ricardo Quaresma (Besiktas), Ronny Lopes (Monaco)