Valencia's new signing Edu is set to miss most of the new season after tearing the cruciate ligament in his left knee during training on Saturday.
Club medical staff said that the 27-year-old Brazil midfielder would require surgery and would probably be out of action for around six months.
Edu twisted his knee as he tried to avoid a challenge from an opponent during a fiercely contested practice match and had to be taken off the field on a stretcher and then to hospital.
Edu joined Valencia last month as a free agent after completing his contract with Premier League side Arsenal.
He played for his new club in their run to the final of the Intertoto Cup and was scheduled to make his domestic debut when the Spanish season kicks off next weekend.
The midfielder moved to Highbury in January 2001 from Corinthians for a reported fee of six million pounds ($10.83 million). He played a key role during Arsenal's unbeaten run to the title in 2004, but took little part last season because of a succession of injuries including a broken toe.
His injury is a major blow to Valencia who have rebuilt their squad following a disappointing season in which they finished seventh in the Primera Liga.
Dutch striker Patrick Kluivert, Spain centre forward David Villa, Uruguay winger Mario Regueiro and Portugal defender Miguel are among the club's other leading signings, while former player Quique Sanchex Flores has been appointed coach.
Valencia will attempt to overturn a 1-0 deficit when they play Hamburg SV in the return leg of an Intertoto Cup final at the Mestalla on Tuesday.
