Marouane Fellaini knee injury a mystery to Man United - sources

Manchester United are struggling to find the cause of the knee pain that is keeping Marouane Fellaini on the sidelines, sources have told ESPN FC.

The midfielder suffered strained knee ligaments playing for Belgium against Bosnia on Oct. 7. After missing six games, he returned in November to feature in four consecutive matches and played 90 minutes in the 1-0 defeat to Basel in the Champions League on Nov. 22. But since coming on as a late substitute against Brighton at Old Trafford on Nov. 25, the 30-year-old has not played.

Jose Mourinho was hopeful Fellaini would be available for the derby with Manchester City but the Belgian was ruled out 24 hours before kick-off after reporting pain in his knee during the final training session.

Fellaini has undergone further scans in a bid to get to the root of the problem but, so far, the injury remains a mystery. And that has left Mourinho and United's medical staff unable to put a timescale on his return to action.

Meanwhile, United are still hopeful they will be able to convince Fellaini to sign a new contract. His current deal will expire at the end of the season and, in theory, could negotiate a precontract agreement with an overseas club from Jan. 1.

United are adamant they will not sell the former Everton man in the January transfer window, even if that means losing him for free in the summer.

Fellaini, who has been linked with clubs in Turkey, Italy and China, turned down an offer of a new contract in September and sources have told ESPN FC he wants a longer deal than the one currently on the table.