The Germany international could be training with his new teammates in the coming days after signing off with a goal with his last touch in a RBL shirt on the weekend.
He told Kicker: "It just felt right for for me to now change clubs, you always say that you should go when it's best.
“It was time to use the momentum and go to a world club. I want to become a figure that you can trust and rely on, just like in Leipzig.
"They not just offered an exciting package, but also gave the best feeling and tried extremely hard for me.
“They actually called a lot and really tried with all their heart to convince me of Chelsea.
“I felt in every conversation that I was really wanted, and I felt that this was the right step. Since I decided to do so a few weeks ago, I have not even regretted it.
“Three years ago, Chelsea in England was exactly what Manchester City was last season and Liverpool are today: champions. In the Premier League, the momentum can quickly change again. I feel that Chelsea is currently building a really strong team for the next few years."