The Denmark defender is set to sign for Barcelona when his contract expires at the end of this month.
He posted a farewell to social media today: "At 16 I had to make a difficult decision: to leave Denmark. There were many interested clubs, but there was only one option for me and that was Chelsea. From the moment I arrived they made me feel at home , and I am very grateful to both Jim Fraser and Neil Bath, who immediately accepted me as part of their family.
"Some of my proudest moments at the club were when we won the FA Youth Cup and the UEFA Youth League. It was a special group of young players that I was proud to have as teammates.
"I was lucky enough to play under fantastic coaches like Conte, Mourinho and now Thomas Tuchel. Last year, the achievement that I am most proud of as a player took place when we lifted the Champions League in Porto.
"I was a child when I arrived at Chelsea with all the hopes and fears of a player starting his career, I am grateful to the whole club for making my dream come true.
"Having spent an incredible ten years at this club, I felt the time was right for a new start for me and my family. Mentally, the last few months have been tough as it has not been an easy decision to leave this club and this support.
"I'm not the most talkative person, so sometimes people don't understand how much the club and the fans mean to me. I will miss the players, the staff and everyone here. I wish the best for the club and the fans in the future."