Mats Hummels admitted that Germany's rot set in over eight months ago after they crashed out of the World Cup with defeat to South Korea on Wednesday.
Two late goals saw Germany eliminated at the group stage of a World Cup for the first time ever as they finished bottom of the group, with Sweden and Mexico progressing to the last 16.
Hummels lamented that Germany's opening defeat to Mexico had left them on the back foot.
"We see in this World Cup that it is easy for any team [to be eliminated], except for Belgium and England. We put ourselves in this situation by losing against Mexico," said Hummels, quoted by L'Equipe.
"There are some things we need to correct, but I will not talk about it publicly. The last time we had a good game, convincing was in October 2017."
Hummels missed Germany's best opportunity to break the deadlock when the game was goalless, with the ball flying wide off his shoulder rather than his head.
After the defeat to Mexico, Germany had restored calm by coming from behind to win against Sweden thanks to a last-minute goal from Toni Kroos.
They were expected to win at a canter against South Korea, who joined them in elimination due to Sweden's 3-0 win over Mexico, but an even game was settled by late goals from Kim Young-gwon and Son Heung-min.
In six previous major tournaments, Germany manager Joachim Low has never failed to reach a semi-final, until now. Under his guidance, Die Mannschaft have won the 2014 World Cup and the 2017 Confederations Cup.
But he will face serious questions about his future, particularly after several questionable selection decisions including the exclusion of Manchester City winger Leroy Sane from the squad.