The German midfield man has accused Blues' medics for "messing with my head".
Ballack was out from April until December with an ankle problem which required two operations.
He told the People: "If I knew I was going to be out for eight months I could have conditioned my mind to cope.
"The club thought it would be four or five weeks at the most but as time went on and there was no improvement it messed with my head. It was very depressing.
"All the time I was being told it wouldn't be too long. I had so many unanswered questions."