The Chelsea midfielder underwent surgery on Monday after coming off in the first half of Saturday's 3-1 win at Sunderland and will now miss the rest of the Premier League season
The 21-year-old had anterior cruciate reconstruction surgery and medial meniscus repair on Monday after being forced off in the first half of Saturday's 3-1 win against Sunderland at the Stadium of Light.
Romeu lasted just 20 minutes on Wearside before being replaced by Oscar. It was hoped the former Barcelona man would be fit enough to fly to Japan with rest of the squad to take part in the Club World Cup.
However, further assessment revealed the injury was more serious than first thought and the Spain under-23 international, who has started four of interim boss Rafa Benitez's five matches in charge, remained in London after it became clear he required surgery to correct the problem.
Romeu has appeared nine times for the Blues this season, scoring once in the Capital One Cup thrashing of Wolves at Stamford Bridge in September, and has racked up 33 games overall since arriving from Barcelona in 2011.