"Following a scan this afternoon (Monday), Ben Youngs (Leicester Tigers) has been ruled out for the remainder of England's NatWest 6 Nations campaign," the RFU said.
"The scans showed the England half back ruptured his MCL (medial collateral ligament) in his left knee and is having further assessment with a surgeon this week.
"He is likely to be out for four months."
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England have called scrumhalf Richard Wigglesworth into a 32-man squad for their next Six Nations game against Wales in place of Youngs, with hooker Luke Cowan-Dickie also added to the squad for the match at Twickenham on Saturday.
Number eight Nathan Hughes will join the camp this week to get his knee injury assessed.
(Reporting by Ed Osmond and Mark Gleeson; Editing by John O'Brien and Ken Ferris)
