Pre-Chelsea career
Juan joined Chelsea from Ajax as a 16 year-old.
Chelsea career
Juan's Chelsea career began tentatively, being used as a bit-part player in the first half of the season as he settled into a new country and a new club. As Jody Morris began to utilise a 3-4-3 formation more often as the campaign went on, Castillo became a force as the left wing-back in that setup, turning in influential performances that helped the club towards a treble success in the Southern section, the Merit Stage, and the FA Youth Cup.
International
Juan has captained the Netherlands at Under-16 level and represented them at the European Under-17 Championships in 2017. He is also eligible to represent the Dominican Republic