The 21-year-old left-back, who has also signed a one-year contract extension with Chelsea, will spend 2021/22 in the Championship at Birmingham City.

Last season he played at AZ Alkmaar where he mostly featured in their second string and then at Den Haag where he made 17 first-team appearances.A former captain of Ajax and the Netherlands at youth level, Castillo, who has also played in more attacking roles, joined Chelsea in 2016 and helped our Academy win the Under-18 Premier League and back-to-back FA Youth Cups.With older age groups, he reached the UEFA Youth League final and scored in an EFL Trophy quarter-final win against Oxford United’s senior team.Prior to last season’s loans in the Netherlands, Castillo spent the 2019/20 campaign back at Ajax where he helped their Under-21s finish fourth in the Dutch second division.His next loan club, Birmingham, are managed by Lee Bowyer who moved there from Charlton in March.