Mourinho left Chelsea just a few weeks into the 2007/08 season having won two Premier League titles in three years, which at the time came as a major shock.
But Ballack – who signed for the Blues in the summer of 2006 – admits he understood why the Portuguese boss left the club.
“I was experienced enough to understand,” Ballack told Sky Sports. “When you are not winning, the coach is more or less the weakest person in the team who can be replaced.
“He was in his fourth year. If you look at the average time a manager has to work in a club, it was quite a good time.
“The way he worked was so intense that maybe you come to a point where things are not working anymore.
“Even then, the fans know what he achieved at the club to take them back to that level.
“It was more or less everyone’s fault – the players’ fault. It was not something we regret, though, we had to look forward.
“The relationship is still good with him. You will not find many players who speak badly about him.”
Ballack went on to win the Premier League title under Carlo Ancelotti in 2009/10, before departing after that title-winning campaign.
“Carlo wanted me [to stay] but the club made a decision to only give players of that age a one-year contract. I wanted two,” he added.
“Today, I can say maybe it was wrong. I should have stayed, even for that one year.
“I could never imagine that I would go back to [Bayer] Leverkusen. Until the last day, I actually thought we would find a way at Chelsea. I was really hoping I could stay until the end.”