The 30-year-old, who has won 13 caps for Germany, said he had believed that the World Cup and his appearances in the national shirt had built him a buffer.
“I was briefly disappointed, because I thought I would have built up a buffer through the World Cup and the games in the national shirt. Apparently that is not the case again.”
Undav said he now has to prove himself again to the Germany coach.
“I have to prove myself anew and give everything anew and prove to a coach again that I belong.
"That is exhausting, proving it to every coach, but I have done it often enough and that’s why I will do it again and hopefully be there in November.”
In the league, the forward is still waiting for a goal involvement after four appearances.
Undav is in his fourth season with VfB Stuttgart after arriving from Brighton. He was named club captain for this new season.