[2] He was promoted to the reserves in July 2014,[4] and made his senior debut on 21 September, coming on as a second half substitute in a 4–1 home loss against Marbella in the Segunda División B championship.

[6] Fabián made his debut in the main category of Spanish football on 23 August 2015, coming on as a substitute for Alfred N'Diaye in a 1–1 home draw against Villarreal.

[8] Upon returning, Fabián became a regular starter under new manager Quique Setién, scoring his first goal in the top tier on 25 September 2017 in a 4–0 home routing of Levante.

[20] Four days later, he made his full debut, starting alongside Sergio Busquets and Dani Parejo in midfield against Sweden, assisting Mikel Oyarzabal's goal in the 87th minute of the 3–0 win.

[25][26][27] In June 2023, Fabián was part of the Spain squad for the 2023 UEFA Nations League Finals and played 16 minutes as a substitute for Mikel Merino in the semi-final against Italy.

[28] On 18 June, he started in the final against Croatia, playing 78 minutes before being substituted for Merino as Spain won 5–4 in the penalty shootout to win their first UEFA Nations League title.

[32] A left-footed central midfielder, Fabián is known for his vision, passing, ball control and dribbling skills, as well as his tactical versatility, and can play in a number of formations, such as 4–4–2, 4–3–3, and 4–2–3–1.