Alonso was named head coach of Bayer Leverkusen in 2022 and completed an unprecedented unbeaten domestic double in 2024, winning the club's first Bundesliga title and first DFB Pokal since 1993.

[14] Their performances attracted the attention of scouts from top Spanish sides and the young Donostiarras separated ways, ending nine years of friendly rivalry, as Alonso went to Real Sociedad and Arteta moved to Catalan giants Barcelona.

Toshack recognised Alonso's potential and invested much time in his young captain, creating a training method designed to improve his touch and control specifically for him.

Alonso revelled in the opportunity to perform in Europe, appearing in all the team's games, and Real Sociedad qualified for the knockout phase of the Champions League.

[11] Madrid failed to meet the £13 million price tag that José Luis Astiazarán, the Real Sociedad president, had placed on Alonso and the deal reached a stalemate.

Alonso's ankle was broken following a tackle from Frank Lampard in Liverpool's 0–1 home defeat against Chelsea on New Year's Day 2005 and the Spaniard was ruled out of action for three months.

[36] Kevin McCarra of The Guardian paid testament to Alonso's skill and dedication to the game, saying, "This marvellously accomplished footballer testified in the Stadio delle Alpi that technique can overcome a serious physical disadvantage.

[39][40] Gerrard returned from injury for the second leg, however, and the captain steered his team to a 1–0 win with the help of a Luis García goal, qualifying for the final against Milan.

While Dida, Milan's acclaimed Brazilian goalkeeper, managed to save the penalty Alonso fired the rebound into the roof of the net, bringing the score to 3–3.

The summer transfer window brought Peter Crouch to Liverpool and the striker's height sparked accusations that the team would change to long ball tactics.

Alonso appeared in all of Liverpool's games in the Champions League but the dominance shown in the previous season had gone as the team lost to Benfica in the knockout stage.

On 7 January 2006, in an FA Cup third-round tie against Luton Town, Alonso assisted Liverpool to a 5–3 comeback victory after being down 3–1 early in the second half.

[54] However, he recovered sufficiently to start the game against West Ham United and Gerrard scored Liverpool's third goal from Alonso's free kick, pulling the team ahead of the opposition.

"[60] The 2007–08 season started well for the Spaniard: Gerrard's absence led to Alonso playing in a more advanced position and he scored twice in a 6–0 rout against Premier League newcomers Derby County.

[70] By the start of the 2008–09 season, neither Alonso or Barry had moved club but the drawn-out transfer saga had left the Spaniard feeling unsettled at Liverpool and unsure of his position in the team.

[75] Statistical analysis reflected Alonso's good form: on 11 December, figures from Opta Sports revealed that he was the first Premier League player to complete 1,000 successful passes in the season.

[85] It was the third time in Alonso's career that he helped his team set a new club record in terms of points gathered, while finishing in second position (he achieved the same feat with Real Sociedad in 2002–03 and with Liverpool in 2008–09).

[86] Readers of Marca made him part of its La Liga team of the season, as their choice defensive midfielder; the only other Real Madrid player featured was Cristiano Ronaldo.

[89] Several members of the Spanish press, as well as a number of Real Madrid supporters, gave Alonso a new nickname during the season: La Barba Roja ("The Red Beard").

Alonso began his third season at Madrid by scoring the second goal in a 2–2 draw against Barcelona in the 2011 Supercopa de España at the Santiago Bernabéu Stadium.

[105][106] He and Bayern captain Philipp Lahm played their final professional game on 20 May, in a 4–1 victory at home to Freiburg in which he assisted the opening goal for Arjen Robben.

Alonso featured mainly as a substitute but, with key players resting, he captained Spain in the final group game against Greece, earning the man of the match award.

[120] Alonso started every game for Spain during the 2010 tournament, playing alongside Sergio Busquets and Xavi in midfield and helping his side to lift their first World Cup trophy.

[127] Alonso made his debut for the Basque Country national team in a friendly against Ghana on 29 December 2001[128] and received regular call ups since, while – due to his busy club schedule – not always being able to appear.

[133] In addition to his creative attributes, he was also capable of excelling as a defensive midfielder due to his strength and powerful physique, combined with his tenacity, tactical intelligence, aggressive tackling and his ability to read the game.

[3][134][135] He was at times criticised, however, for occasionally committing rash challenges and for his tendency to lose his composure and pick up unnecessary cards for particularly hard fouls.

[136] Regarding Alonso's playing style, Jonathan Wilson noted in a 2013 article for The Guardian that he was an example of a more creative interpreter of the holding midfield role, who, "although capable of making tackles, focused on keeping the ball moving, occasionally raking long passes out to the flanks to change the angle of attack like an old-style regista.

[153][154] During the summer transfer window, he signed players including Switzerland captain Granit Xhaka, Jonas Hofmann, Alejandro Grimaldo and Matej Kovar.

Alonso set his team up in a 3–4–3 formation with an emphasis on passing and counter-attack, while also only conceding five goals from open play in their first 11 Bundesliga fixtures.

[168][169] In March 2008, instead of travelling with the team, Alonso stayed in Merseyside to be at his wife's side while she gave birth, saying, "It was a little frustrating to miss the match against Inter but I have to be with my family at times like these.