Gerard Pique suffered knee ligament damage during Saturday's 1-0 defeat to Atletico Madrid, Barcelona have confirmed.
Pique hobbled off in clear discomfort at the Wanda Metropolitano after Angel Correa landed awkwardly on his lower right leg.
Late on Saturday, Barca announced the experienced centre-back had suffered a sprain and further analysis revealed damage to both the interior lateral and anterior cruciate ligaments.
"Tests carried out this Sunday on Gerard Pique have shown that he has a grade three sprain in the internal lateral ligament and partial injury to the anterior cruciate ligament in his right knee," a statement read.
"The evolution of the injury will condition his availability."
Although Barca declined to put a timeframe on Pique's recovery, Catalan paper Sport reported that much hinges upon whether or not surgery is required.
A sprain allowed to heel on its own might see Pique back in action within two months, while going under the knife would see him sidelined for between four to six months.
The bad news continues to mount for boss Ronald Koeman, who will also be without the versatile Sergi Roberto for two months due to a ruptured thigh muscle.
Barca are languishing in midtable in LaLiga after eight games, with Roberto and Pique joining Sergio Busquets, Ansu Fati and Samuel Umtiti in the treatment room.