In May 2011, Bustinza was promoted to the reserves in the Segunda División B. He appeared regularly for them the following campaigns, scoring a career-best five goals in 2012–13.
On 15 July 2014, Bustinza was promoted to the Basques' main squad in La Liga. He made his debut in the competition on 25 August, coming on as a late substitute in a 1–0 away win over UD Almería.
Bustinza was handed his first start for the Lions on 2 May 2015, being sent off in the 93rd minute of the 0–0 draw with Atlético Madrid for two bookable offences. Later that month, he played the full 90 minutes in the final of the Copa del Rey against FC Barcelona, a 3–1 loss.
Leganés
On 22 July 2015, Bustinza was loaned to Segunda División side CD Leganés in a season-long deal. The following 18 July, having contributed 1,134 minutes in all competitions to a top-flight promotion, he signed a permanent three-year contract after cutting ties with Athletic.
Bustinza scored the first of two goals in the Spanish top tier on 21 February 2018, but in a 1–3 home defeat to Real Madrid.
Málaga
On 2 July 2022, free agent Bustinza joined second-tier Málaga CF on a three-year deal. On 30 June 2023, after suffering relegation, he activated a clause on his contract and left.
Amorebieta
In November 2023, Bustinza started training with SD Amorebieta. On 18 January 2024, he agreed to a deal at the second-division team.
International career
Bustinza made his debut for the unofficial Basque Country national team in May 2019, in a 0–0 draw away to Panama for which a small, youthful and inexperienced squad was selected.
↑Funqueiro, Lucía (24 June 2022). "El Málaga pone el ojo en Bustinza"[Málaga set their sights on Bustinza] (in Spanish). Grada 3. Retrieved 4 July 2022.
↑González, Raúl (19 July 2016). "Unai Bustinza, el soldado de Butarque"[Unai Bustinza, the soldier of Butarque] (in Spanish). Sphera Sports. Retrieved 27 February 2024.
↑Guerreiro, Daniel (9 November 2023). "Bustinza se entrena con el Amore"[Bustinza trains with Amore]. Diario AS (in Spanish). Retrieved 18 January 2024.