After scoring six goals in only seven appearances for the B's in the beginning of the 2013–14 season, Guillermo was called up to the first team by coach Ernesto Valverde.[3] On 21 October 2013, he was included in the squad for a 2–0 La Liga home win against Villarreal CF, but did not leave the bench. He finally made his debut in the competition on 9 November, starting and playing 68 minutes in a 2–1 victory over Levante UD also at San Mamés.[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.[6] His second came on 21 October in the group phase of the UEFA Champions League, in a 2–1 away loss to FC Porto.[7]
On 13 August 2015, Guillermo was loaned to CD Leganés of Segunda División in a season-long deal.[8] He scored his first goal ten days later in the 3–2 defeat at UD Almería,[9] adding five more in all competitions in an eventual promotion.[10][11]
On 17 August 2016, Guillermo terminated his contract with Athletic and signed for two years with Elche CF later that day.[12] The following 27 June, after suffering relegation, he agreed to a three-year deal at fellow second tier club CD Numancia.[13]
^Martín, Javi (27 August 2012). "Guillermo llama a la puerta del nueve" [Guillermo knocks on nine's door] (in Spanish). La Cantera da Lezama. Retrieved 7 September 2023.
^Velasco, Juanma (17 August 2016). "Guillermo Fernández se une al Elche" [Guillermo Fernández joins Elche]. Marca (in Spanish). Retrieved 17 August 2016.