Undav will start the friendly against Finland on Sunday as head coach Julian Nagelsmann goes for as close to his first-choice XI as possible.
With Kai Havertz involved in the Champions League final with Arsenal, Undav gets the nod in attack.
The former Union St Gilloise striker, 29, scored eight goals for Albion without ever really imposing himself as a first-choice.
He has since excelled with Stuttgart and is poised to sign a new deal.
Pascal Gross, whose move from Dortmund to Albion in mid-season has helped him regain his national squad place, is likely to have a support role at the World Cup.
The German national team will fly from Frankfurt to Chicago on Tuesday afternoon ahead of their warm-up match against hosts USA on Saturday.
Ahead of the Finland game, Nagelsmann said: "We don't have that much time.
“We are trying to get into a rhythm quickly.
“We will see many players who could also start in the first World Cup match.”
But he added: "The competition for places is there."
Germany are in the same group as Curacao, who lost 4-1 to Scotland in their penultimate warm-up game after Jurgen Locadia was sent off, as well as Ecuador and Ivory Coast.