A native of Ferrol, Carreras spent five years at the Racing de Ferrol academy before moving on to the Deportivo de La Coruña academy. It was at Deportivo that he was converted into a left-back. In 2017, at the age of 14, he left Deportivo and joined the Real Madrid academy before signing for the Manchester United academy in 2020. At United, he was promoted to the senior squad but never made a single appearance. He was loaned to Benfica in 2023 before they made his move permanent in 2024 for €6 million.
Carreras had his breakthrough season at Benfica in 2024/25, where he has played 42 games so far, scoring four goals and assisting another four.