Midfielder Vanaken has scored three international goals this year, while De Ketelaere made his only Belgium appearance in November 2020.
Brugge have enjoyed a strong start to the season, drawing with Paris Saint-Germain and beating RB Leipzig in the Champions League.
Belgium face world champions France in the second Nations League semi-final on October 7 in Turin, with the winners to take on either Italy or Spain in the final three days later in Milan.
"Hans Vanaken is there because he is performing very well every time he is on the pitch at the moment," said Martinez, who is without senior players including Thomas Vermaelen and Dries Mertens.
"As for Charles De Ketelaere, now is the right time for him to take a step forward in the national team, especially in the absence of (Jeremy) Doku and Mertens."
Belgium, the world's number one-ranked side, have key attacking players Kevin De Bruyne, Eden Hazard and Romelu Lukaku all fit.
Goalkeepers: Koen Casteels (Wolfsburg/GER), Thibaut Courtois (Real Madrid/ESP), Simon Mignolet (Club Brugge), Matz Sels (Strasbourg/FRA)
Defenders: Toby Alderweireld (Al-Duhail/QAT), Dedryck Boyata (Hertha Berlin/GER), Jason Denayer (Lyon/FRA), Jan Vertonghen (Benfica/POR), Yannick Carrasco (Atletico Madrid/ESP), Timothy Castagne (Leicester/ENG), Thorgan Hazard (Borussia Dortmund/GER), Thomas Meunier (Borussia Dortmund/GER), Alexis Saelemaekers (AC Milan/ITA)
Midfielders: Kevin De Bruyne (Manchester City/ENG), Leander Dendoncker (Wolves/ENG), Youri Tielemans (Leicester/ENG), Hans Vanaken (Club Brugge), Axel Witsel (Borussia Dortmund/GER)
Forwards: Michy Batshuayi (Besiktas/TUR), Charles De Ketelaere (Club Brugge), Eden Hazard (Real Madrid/ESP), Romelu Lukaku (Chelsea/ENG), Dodi Lukebakio (Hertha Berlin/GER), Leandro Trossard (Brighton/ENG).