While on loan, he scored two goals in 11 appearances and helped the club win the Segunda Division title and secure promotion to Uruguay's top flight.

[citation needed] On 27 July 2018, Castellanos joined Major League Soccer side New York City FC on loan until the end of the 2018 season.

[citation needed] In 2020, Castellanos declined in his production, scoring six goals and adding three assists in 22 matches during a season shortened by the COVID-19 Pandemic.

During the first round of the 2020 MLS Cup Playoffs Castellanos had his initial penalty in a shootout vs. Orlando City saved, but VAR overturned the call due to goalkeeper Pedro Gallese leaving the goal line early.

[citation needed] On 25 July 2022, Castellanos joined newly promoted La Liga club Girona on a season-long loan.

[13][14] In October 2019, Castellanos was called up for the first time to the Argentina national under-23 football team by coach Fernando Batista for two friendly matches against Mexico.

[16] In August 2024, Castellanos received his first call-up for the senior national team for the games against Chile and Colombia for the 2026 FIFA World Cup qualifiers.