[2] Born in Barakaldo, Biscay, del Horno began his career at Athletic Bilbao in 1999, where his combative tackling and strength in the air saw him rise through the ranks as a left-footed central defender.

[5] The following 22 February, late into the first half of a UEFA Champions League round-of-16 clash at home against FC Barcelona, he received a straight red card for a foul on Lionel Messi, as his team went on to lose the match 1–2 and the tie 2–3.

Athletic Bilbao had expressed interest, but withdrew from further negotiations, forfeiting any further financial advantage; following surgery on his injured Achilles heel, which kept him off the pitches for most of the season, he made his debut for his new team on 3 March 2007 in a 1–0 home win over RC Celta de Vigo.

[13] At the Che, his physical problems persisted[14] and, after an unassuming first half of 2009–10 – only played in the Copa del Rey and only when facing rivals of smaller entity – he was loaned to Real Valladolid until the end of the season, on 30 January;[15] an undisputed starter since his arrival, he could not however prevent the Castile and León side from returning to Segunda División, after a three-year stay in the top division.

[23] Medical staff in the Royal Spanish Football Federation alleged that Chelsea had prior knowledge of the former's injury, but allowed him to continue playing; subsequently, his vacant position was controversially filled in by naturalised Argentinian Mariano Pernía, of Getafe CF.