Chelsea have confirmed that Ramires will be sidelined for 'three to four weeks' with a medial knee ligament injury.
The midfielder was subsituted with the problem during Saturday's FA Cup win at Queens Park Rangers.
Manager Andre Villas-Boas said after the match that he expected the 24-year-old to be out for a month.
Ramires will miss Chelsea's Premier League match at home to Manchester United on Sunday, as well as the first leg of their Champions League last 16 tie at Napoli.