Born in Urduliz, Biscay, Basque Country, Merino graduated from Danok Bat CF's youth academy after spells at Athletic Bilbao's Lezama and American team Javanon FC.

[3] On 29 June 2015, after scoring a career-best 18 goals during the season as they returned to Segunda División after a 19-year absence,[4] he was promoted to the main squad in La Liga.

[5] Merino made his official debut with the first team on 6 August 2015, coming on as a second-half substitute for Javier Eraso in a 0–0 away draw against Inter Baku PIK in the third qualifying round of the UEFA Europa League.

[7] Merino started in the 4–0 home rout of FC Barcelona in the Supercopa de España on 14 August 2015 as a left winger, providing the assist for Aritz Aduriz's second goal.

[13] He scored four times in his first four matches with the club, surpassing previous holder Bebeto who held the distinction since 1992;[14] this feat earned him January's Segunda División Player of the Month award.