Soccer-Barca's Rafinha to have surgery on injured knee

* Midfielder damages cruciate ligament in right knee * Roma's Nainggolan says regrets causing injury * Brazil's Rafinha may miss the rest of the season (Adds Nainggolan Tweet) MADRID, Sept 17 (Reuters) - Barcelona midfielder Rafinha will undergo surgery after damaging the anterior cruciate ligament in his right knee in Wednesday's 1-1 Champions League Group E draw at AS Roma. The Brazil international had only been on the pitch for a few minutes when he was injured by a robust tackle from behind by Roma midfielder Radja Nainggolan. Barca said on their website (www.fcbarcelona.es) on Thursday Rafinha, the younger brother of Bayern Munich midfielder Thiago Alcantara, would have the operation in the next few days. They did not say how long he might be sidelined but Spanish media reported he would miss the rest of the season. Nainggolan, a Belgium international, said on his Twitter feed on Thursday he had not meant to hurt Rafinha. "Hi @rafinha, I am really sorry about the bad news," Nainggolan wrote. "It was not my intention to hurt you," he added. "I hope to see you on the pitch soon, a big hug." Rafinha is not an automatic starter for Barca coach Luis Enrique but has been on good form in recent weeks and will be a significant loss for the Spanish and European champions. Their next outing is a La Liga game at home to Levante on Sunday and they host Bayer Leverkusen in the Champions League at the end of the month. Rafinha and Thiago are the sons of former Brazil midfielder Mazinho, who was part of the 1994 World Cup-winning side. Thiago, who plays for Spain and like Rafinha is a graduate of Barca's academy, needed two operations on a similar injury he sustained in March last year and missed the 2014 World Cup finals in Brazil. (Reporting by Iain Rogers, editing by Amlan Chakraborty and Pritha Sarkar)