The goalkeeper joined the Blues in an €80m deal this week and he took to social media to convey his goodbyes to the club and fans.
“Today is the day I leave my Athletic home and it’s a day of mixed feelings to leave a club I joined as a nine-year-old,” he wrote.
“It’s where I grew up as a person and a footballer, from when I was little, and I understand some fans will understand my decision and others won’t.
“But I want to let everyone know I have taken it in a relaxed manner and with responsibility. It was not easy to leave everything for which I have worked for so many years, the effort put in and the experiences.
“However, sometimes in football you have to take a step forward to face personal challenges and develop your career.
“I’ve analysed what is best for me so I consider the opportunity presented to me by signing for Chelsea is best for my future.
“That doesn’t mean I forget Athletic’s values, the same ones I’ve been cultivating for 15 years, and for that I have to thank the club and its supporters.
“I want to thank Athletic for the love and respect they have instilled in me during these years and I’ll always take that with me.
“To all my teammates over the years and the ones I leave behind in the dressing room – you are the best. “And finally, thanks to the fans. Every time I walked into San Mames I felt privileged and I noted your love and strength. “I will always be in debt to what you have made me feel. Aupa Athletic!!”
By: Benjamin Owusu