The midfielder has stressed there's no reason to worry about the centre-back after he was dropped from the Germany starting XI
The centre-back has lost his starting berth with the national team to Bayern Munich's Jerome Boateng, but the former Nurngberg midfielder is confident that the recent events will only benefit his colleague on the long term.
"I talked with Mats before the World Cup qualifier against Faroe Islands. It's not an easy situation for him," Gundogan told Ruhr Nachrichten.
"However, Mats is still one of the best centre-backs in the world in my opinion. He has helped BVB forward in the past few years and is a key figure for us.
"Every player goes through difficult periods like this. It happened to me as well. Mats is a strong and intelligent guy and I'm sure he will draw the right conclusions from all this.
"I am not the least worried about him."
Hummels has made 25 international appearances for Germany so far, scoring one goal.