He made his Serie A debut with the side on 7 February, starting in a 1–2 away loss against Fuerza Amarilla, and scored his first goal in the category late in the month, in a 1–1 home draw against Mushuc Runa.
On 29 August 2021, Castillo was called up to the Ecuador national team by manager Gustavo Alfaro for three 2022 FIFA World Cup qualifiers against Paraguay, Chile and Uruguay.
[14][15] FIFA closed the investigation after nearly one month on 10 June, confirming Ecuador's spot in the World Cup, saying he was an Ecuadorian citizen and never represented Colombia in a competitive match.
[16] Spanish sporting newspaper Marca published audio that was apparently of Castillo admitting his illegitimate entry into Ecuador, as well as his birthplace in Colombia, and his correct year of birth.
The Ecuadorian Football Federation (FEF) were however, fined CHF 100,000 and handed a 3-point deduction in the 2026 World Cup qualifiers for CONMEBOL due to the use of a document which contained false information.