The Stamford Bridge midfielder suffered the nasty injury during Chelsea's 3-1 win over Sunderland at the Stadium of Light on Saturday, and the Spaniard returned to London after the match rather than travelling to Japan for the Club World Cup.
Romeu had surgery on the damaged anterior cruciate and his medial meniscus on Monday, and the west London club expects the midfielder to be out for six months.
The former-Barcelona youth team player joined the Blues in a £4.35m deal in August 2011, and has gone on to make 33 appearances for the European champions.
Chelsea boss Rafa Benitez now faces a selection problem, with a shortage of defensive midfielders.
John Obi Mikel served the first of his three-match ban against Sunderland, while Michael Essien is currently half-way through a season-long to Real Madrid.
Former-Chelsea boss Roberto Di Matteo also sold Raul Meireles to Fenerbahce in the summer.
On Saturday, Benitez called on Brazilian playmaker Oscar to replace the injured Romeu, but the former-Internacional star is unlikely to serve as a like-for-like replacement for the crocked midfielder.
The Blues have an extremely busy fixture list between now and the New Year, with eight games from 13 December to 5 January, including two in Japan.
Benitez had initially hoped Romeu would be able to join up with the squad in Yokohama ahead of their semi-final against Mexican side Monterrey, but the Spaniard's injury was too serious, and the club decided surgery was the best option.