Barcelona have confirmed that Rafinha has torn his right anterior cruciate ligament.
Rafinha, 22, sustained the injury during Wednesday's 1-1 Champions League draw with Roma, leaving the game on a stretcher after being on the receiving end of a heavy challenge from Roma midfielder Radja Nainggolan.
The Brazil international was forced off the pitch just two minutes after coming on as a substitute for his side midway through the second half.
Barca boss Luis Enrique said after the game that the injury "doesn't look good," and the club confirmed on Thursday the extent of the damage after the player underwent tests.
- FC Barcelona (@FCBarcelona) September 17, 2015
A statement on Barca's official website read: "Tests this morning have revealed that FC Barcelona midfielder Rafael Alcantara -- also known as 'Rafinha' -- has suffered a tear to the anterior cruciate ligament in his right leg. He will undergo surgery in the coming days."
According to reports in Spain, Rafinha is set to be out for at least six months with the injury.
"It is a very ugly injury," Luis Enrique said after the Roma game. "That is the bad news from this round.
"The injury has affected the game, and the spirit of the players. It was already difficult in the first half, with their goal, and then the second half with the injury."
- barcastuff (@barcastuff) September 17, 2015
The news will come as a blow to Barca, with Rafinha having featured in six of their seven games this season, while scoring in their UEFA Super Cup victory last month.
Nainggolan tweeted an apology on Thursday, saying: Hello @rafinha, I am sorry about the bad news, I didn't mean to hurt you. I hope to see you soon on the field, hugs!
Hola @rafinha, lo siento mucho por la mala noticia, no fue mi intención hacerte daño. Espero verte pronto en el campo, un fuerte abrazo!
- Radja Nainggolan (@OfficialRadja) September 17, 2015
Despite the injury, Catalan newspaper El Mundo Deportivo reported on Thursday that Barca still plan to renew the midfielder's contract until 2021.
Barcelona cannot sign any new players until Jan. 4 because of the FIFA transfer ban.