The Athletic Club forward underwent medical tests on Monday after sustaining the injury on Saturday, with the diagnosis confirming damage to the ligament in his left knee.
Guruzeta now faces surgery, with the date to be confirmed by the club. A recovery period of six to eight months is anticipated, which would leave Athletic without one of their principal attacking options for the remainder of the campaign.
The injury is particularly cruel because the 30-year-old has already endured the same setback. In 2019, while playing for Bilbao Athletic after making his senior-team debut, he ruptured the anterior cruciate ligament in his right knee.
He subsequently rebuilt his career and returned to the first team, becoming an important part of Ernesto Valverde’s side. His movement, finishing and ability to link play have allowed him to operate as a central striker or drop into deeper positions to help create chances.
Guruzeta has made 186 appearances for Athletic’s first team, scoring 48 goals. He contributed 17 goals in 48 appearances last season, but had yet to find the net in five outings this term after returning from a muscular injury.
His absence is a significant problem for head coach Edin Terzić, who had been working the striker back towards full fitness after his earlier setback. Guruzeta had been building his rhythm as Athletic adjusted to Terzić’s methods, and the new injury interrupts that progress at a difficult point in the season.
Athletic have limited senior alternatives at centre-forward. Iñaki Williams and Maroan are among the options available to Terzić, although the coach may need to adapt the roles of other attackers or turn to players from the reserve team who have trained with the first team.
The timing adds to the challenge. The Copa del Rey has yet to begin, while Athletic have played only seven full league matches, leaving a long stretch of fixtures ahead without Guruzeta.
A move for a replacement in the winter transfer window is not an immediate certainty. The club will have to weigh its financial position and the available market against the needs of a team that has already lost an experienced attacking presence.
For Guruzeta, the immediate priority is surgery and the difficult rehabilitation that follows. Having overcome a serious knee injury once before, he knows the demands of the road ahead, but Athletic must now plan for a prolonged period without one of their most established forwards.