By FOX Sports staff
Although he has spent the last couple of seasons regarded by many as one of Europe's top defenders, Hummels has had a disappointing run of form in the last few months as a number of uncharacteristic mistakes has seen him lose his place in the Germany starting XI to Bayern Munich's Jerome Boateng.
However, Gundogan has leapt to the defence of his club and country colleague by claiming the 24-year-old deserves credit for the role he has played in Dortmund's recent successes.
"I talked with Mats before the [2014 FIFA] World Cup qualifier against Faroe Islands," he told Ruhr Nachrichten. "It's not an easy situation for him.
"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."
The ex-Nuremberg midfielder, who is also seen by many as an important player for Dortmund, also believes these recent problems will be no problem for Hummels to handle.
"Every player goes through difficult periods like these," Gundogan added. "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 began his career at Bayern but signed for Dortmund in 2009 after initially joining the club on loan, and has since gone on to earn 25 caps for Germany.