Dhaka, Sept. 16 -- Athletic Bilbao suffered a major blow on the eve of their long-awaited Champions League return, as midfielder Benat Prados has been ruled out for the season with a serious knee injury.

The Basque club confirmed Monday evening that the 24-year-old tore the anterior cruciate ligament in his left knee during training last week. "In the coming days we will decide the date for surgery," Athletic said in a statement.

A product of Athletic's famed youth system, Prados made 45 appearances last season and 33 the year before, following his loan spell at second-tier Mirandes.

Though considered the third-choice midfielder under coach Ernesto Valverde-behind Mikel Jauregizar and the injury-prone veteran Inigo Ruiz de Galarreta-Pr...