Athletic Bilbao midfielder Tiko has been sidelined for at least six months after it was confirmed that he injured his cruciate ligament at the weekend.
The 30-year-old was playing for the Basque Country representative team in the 2-2 draw with Catalunya in Camp Nou on Sunday, when he went down unchallenged in the 72nd minute.
Tests have now revealed that he has damaged ligaments in his right knee and will require surgery.
Tiko entered the game as a 55th-minute substitute, but was stretchered off again 17 minutes later and will now be out of action until the final weeks of the season.
"We have done an examination and that has confirmed the diagnosis that the Real Sociedad doctor, Gonzalez de Suso, who was at the game, gave us," said club doctor Paco Angulo.
"The player has a break in the cruciate ligament of his right knee. There are also other slight injuries around the area, which are normal when an injury of this type occurs.
"Surgery will be required after one month, leaving time for the knee to dry off and for the current swelling to go down.
"The normal time frame for recovery from something like this is six or seven months."