The Daily Mail says the winger has damaged his left medial ligament so will miss at least another four weeks.
United manager Sir Alex Ferguson hoped to include Young in his squad to face Tottenham last weekend but the winger was ruled out after medical tests.
The midfielder will definitely miss both legs of the Champions League second round clash against Jose Mourinho's Spanish giants Real. Young is also likely to be absent for the domestic fixtures against Fulham, Southampton, Everton, QPR, Norwich, West Ham and Reading.
He will be unavailable for England's friendly against Brazil, but should be fit for the crucial March World Cup qualifying double header with San Marino and Montenegro.