The Germany international says that mental health problems hampered his build-up to the new season and thanked manager Thorsten Fink for his understanding at a difficult time.
The 25-year-old received the support of his manager Thorsten Fink, who agreed to a request from the player for his condition to be investigated.
"At the end of the pre-season, I found it harder and harder to get up," Aogo told Bild.
"Body and mind are interrelated. I thought that it was just a phase but the point came when I could no longer help the team.
"I was depressed and mentally I felt that I was no longer myself. Therefore, I asked the coach if I could get it checked thoroughly."
Aogo spent over a week receiving treatment and said he is relieved to be back to his normal self again.
"After 10 days [of care] the motivation and momentum came back. This was not an easy time but I feel really good again."