Mats Hummels says returning to Borussia Dortmund so soon after his return to Bayern Munich felt "very strange" and is confident his performances will improve.
Hummels, 27, left Dortmund this summer after making 225 Bundesliga appearances for the Schwarzgelben from January 2008 through to May 2016, and returned to the club he left as a 19-year-old.
On Sunday, he was back at the Westfalenstadion as Bayern secured a 2-0 win over Dortmund in the German Super Cup and received a hostile reception from the home fans.
The Germany international, who has said he had expected to be whistled on his return, told reporters that "it felt very strange" to play in the stadium as an opponent.
"It was an unusual feeling for me, and when we passed through the stadium gates, my emotions took hold of me," he said. "I was in a state of turmoil, if you want to put it that way."
Hummels, like Bayern's other Euro 2016 participants, had only resumed training following the conclusion of the club's tour of the United States and had therefore only begun preparing for the new campaign a week before the Super Cup.
Playing in an unfamiliar role as the right-sided centre-back alongside Javi Martinez, the former Dortmund captain appear to lack understanding with his new teammates and was caught out of position during the first half.
"It was an important match to make another step physically, but performance-wise it was hopefully not the end of the road for me," Hummels said.
Hummels has played to the left of new Bayern teammate Jerome Boateng for Germany in recent years.