Manchester United captain Nemanja Vidic has been ruled out for eight weeks following a knee operation.Vidic missed Sunday's Premier League win at Liverpool after complaining of tightness in his right knee.However, further tests revealed the problem to be more severe than initially expected and the centre-back was forced to undergo surgery.The 30-year-old Serb missed the final five months of last season with a ruptured cruciate knee ligament."Nemanja Vidic has had a meniscus operation on his right knee and will be out for around eight weeks," read a club statement."Manchester United manager Sir Alex Ferguson rested Vidic as a precautionary measure at the weekend after he complained of tightness in his knee. However, further medical investigation revealed the underlying problem."The central defender was operated on this week and will be sidelined for about eight weeks."It is the latest defensive blow for United, who have suffered injuries to fellow centre-backs Chris Smalling and Phil Jones.Smalling was sidelined for 10 weeks in July with a broken metatarsal and Jones for eight weeks in early September with a knee complaint.Vidic joined United from Spartak Moscow in 2006 and has scored 17 goals in 246 appearances for the Old Trafford club.