The Brazilian underwent a scan today and his club have confirmed that he is expected to be out for three to four weeks.
Ramires suffered the injury as he blocked a shot from Clint Hill and went off on a stretcher as his side held on to claim a 1-0 victory.
He will miss Chelsea's match against Manchester United at Stamford Bridge next Sunday as well as the club's trip to Napoli for the first leg of their Champions League last 16 clash on February 21st.
Speaking after yesterday's game manager Andre Villas-Boas was hopeful that Ramires would avoid an operation and today's scan has proved that to be the case.
"It looks like a medial ligament injury, not so bad in terms of knee injuries," he said.
"He will have the MRI scan tomorrow, I think hopefully he will avoid an operation. We could look at four weeks [out]. We have to see the full extent of the injury."
Ramires has made 26 appearances for the Blues so far this season, netting eight times.