Boca Juniors defender Gabriel Paletta has been ruled out for around six months after tearing the cruciate ligament in his right knee.
The 22-year-old picked up the injury in Sunday's 1-0 Apertura victory over Arsenal and MRI scans today confirmed the seriousness of the problem.
Club doctor Jose Veiga said: "When the Arsenal match ended, we saw he had a knee twist so we ordered him to undergo some scans. Today, the results revealed he tore cruciate ligaments."
The news is a blow to Boca's hopes of claiming the Apertura title. They currently lead the table along with San Lorenzo with five games to play.
Boca also lost leading scorer Martin Palermo to a similar injury back in August.