The 22-year-old penned a five-year deal with the Potters in June before representing Nigeria at the 2018 Fifa World Cup.
Etebo played in every of the Super Eagles' games and gave impressive performances in Russia.
Following his country's early exit in the group stage, the midfielder was given an extended break before joining his new club in London on Monday.
Etebo joined the team in time for their flight to Herzlake while Senegal duo Badou Ndiaye and Mame Diouf are expected to reunite with their teammates on Wednesday.
The Potters will play German Bundesliga II side Bochum at the OBI Arena on Wednesday before participating in a three-team tournament that features SV Meppen and Hamburger SV on Saturday.