The German has gone back to his homeland for the last major move of his career.
The veteran has signed for Borussia Dortmund and will no longer be in the Premier League.
In a video message, he said: “I am sorry I couldn’t say goodbye in the right way.
“Hopefully I can come back soon to a game in the stadium and say goodbye to all of you.
“The last seven years have been really special to me, the way you supported me from the first day until the last day.
“I think we had a special time with staying in the Premier League for that long and the European qualification.
“Last year with all our European trips, it was special for the club.
“It was an honor to play for the club.
“I really enjoyed it from the start. I grew with the club.
“I really appreciate your support even now.
“I wanted to play for my boyhood dream club, Borussia Dortmund, which is a really proud moment for me and my family.
“I will support you like a fan. I will watch games.
“Hopefully I see you soon and I can say goodbye. All the best and good luck. Thank you.”