Spaniard Romeu suffered an anterior cruciate knee ligament injury during the first half of the league match at Sunderland on Saturday, ruling him out of Chelsea's trip to Japan for the Club World Cup this week.
"Oriol Romeu underwent anterior cruciate reconstruction surgery and medial meniscus repair yesterday. He is expected to be out for six months," a statement said.
Romeu, who played in the London Olympics for Spain, has struggled to establish himself at Chelsea since joining from Barcelona in 2011, making only 16 league appearances to date. (Reporting by Martyn Herman; Editing by Clare Fallon)