Seven years later, he was promoted to the reserves in the Segunda División B (bypassing CD Basconia, the farm team and usual step in the progression of young players).

[4][5] Guillermo scored his first goal for the Lions' main squad on 23 February 2014, coming on for Iker Muniain in the 79th minute of an eventual 2–0 win at Real Betis and netting through a header after 60 seconds.

[13] On 10 January 2018, Guillermo scored twice to help to a 2–2 draw against Real Madrid at the Santiago Bernabéu Stadium for the Copa del Rey after having come as a first-half substitute, but the hosts had already won 3–0 in the first leg to progress to the quarter-finals.

[15] Guillermo alternated between the second and third divisions the following seasons, representing Burgos CF – promoting to the former in 2021[16][17]– Gimnàstic de Tarragona[18] and Cultural y Deportiva Leonesa.

[19] He moved abroad for the first time in his career in July 2024, with the 31-year-old signing with Indian Super League side NorthEast United FC.