[10][11] Another record fell on 30 August 2009, as Muniain started in the 1–0 home victory over RCD Espanyol, 2009–10's opener, becoming the youngest player to have donned the club's shirt in La Liga (overtaking Patxi Ferreira).

[12] Two weeks later he scored again in European competition, 3–0 at home against FK Austria Wien, after a good team move involving Andoni Iraola and veteran Joseba Etxeberria.

[13] On 4 October 2009, Muniain became the youngest player to score in a first division match as he achieved that feat in a 2–2 draw at Real Valladolid, aged 16 years and 289 days[14][15]– that record was surpassed by Málaga CF's Fabrice Olinga in August 2012.

[27] Eighteen days later, he celebrated his 21st birthday by grabbing a brace in a 4–0 domestic cup home victory against RC Celta de Vigo, which signified his team progressed through to the round of 16 4–1 on aggregate.

[29][30] On 4 April 2015, during a game against Sevilla FC, Muniain suffered the first serious injury of his career, being sidelined for several months with an anterior cruciate ligament ailment to his left knee[31]– this meant he was unable to take any part in the year's Spanish Cup or Supercopa de España finals.

[35] Muniain successfully regained his first-team place in December 2015 and made 86 further appearances between then and 28 September 2017, when he suffered another serious ACL injury (this time to his right knee) in the closing stages of a Europa League fixture at home to FC Zorya Luhansk, potentially ending his season.

Athletic lost the first match 1–0 to Real Sociedad,[48] with Muniain breaking a footballing superstition before kick-off by touching the trophy (considered to be 'bad luck')[49] but praised for his sportsmanship for staying on the pitch after the final whistle to applaud the winners.

They are won on the pitch, and that is the only reality", but this time it was noted he kept his hands well away from the prize;[51] nevertheless his team were defeated 4–0 by Barcelona and he was withdrawn at half-time with the score at 0–0[52] and did not feature again that season, missing the last eight league fixtures.

On 31 October 2021, Muniain scored a late equaliser in the Basque derby via a free kick and was again conspicuous for his sportsmanship in consoling the opposing goalkeeper Álex Remiro, a former Athletic player whose error had contributed to the goal.

[54] Despite knocking both Barcelona and Real Madrid out of the domestic cup, they lost in the semi-finals to Valencia, and finished the league campaign in eighth, just out of the European qualification places.

[59][60] After a promising start under returning coach Ernesto Valverde the team again finished eighth and lost narrowly in the cup semi-finals (this time to Osasuna);[61] he featured regularly but became less of a habitual starter, coming off the bench in 11 of his 30 league appearances.

[62] Athletic also began 2023–24 strongly, but Muniain was selected rarely during the first half of the season, with Valverde opting for Oihan Sancet as the starting attacking midfielder and either Álex Berenguer or rookie Unai Gómez as an offensive substitute.

[63] On 30 September against Real Sociedad, he reached 542 appearances for the club, surpassing Txetxu Rojo's record for an outfield player (albeit with far fewer minutes overall)[64][65] and trailing only Iribar.

[67] Although there was some expectation that he may remain for the upcoming European campaign,[68] on 24 April 2024 Muniain announced he would leave Athletic on 30 June, marking the end of his 15-year tenure in which he became the second-highest appearance maker in the club's history; he declared himself happy with the timing of his departure having won the cup,[69][70] and it was speculated that he would move abroad to continue his playing career.

[90] He is an admirer of Sergio Agüero, and named his pet Labrador Kun after the Argentina international; Muniain's own nickname, Bart Simpson, was given to him by Fernando Amorebieta due to his mischievous personality as well as his diminutive stature and fair hair.