Manchester United midfielder Marouane Fellaini is reportedly on the sidelines due to a "mystery" knee injury.

The Belgium international last featured for the Red Devils when he came off the bench in his side's 1-0 win over Brighton & Hove Albion on 25 November. He has been out of action due to the injury.

According to ESPN, United are struggling to find the cause of the knee pain that has forced Fellaini to spend his time away from the pitch. He suffered a medial ligament injury while in action for Belgium against Bosnia during the international break in October.

Fellaini recovered from the injury and returned to action in United's 1-0 defeat to Chelsea at Stamford Bridge on 5 November. He went on to make four appearances in the same month, which included a full 90 minutes in his side's 1-0 defeat to Basel in the Champions League.

The former Everton midfielder was expected to be fit for the Manchester derby clash against Pep Guardiola's Manchester City. However, a pain in the knee in the final training session forced Jose Mourinho to omit him from the squad.

He has undergone further scans to find out the cause of his knee pain, but it has remained a mystery, according to ESPN.

Lack of clarity on the cause of the injury has forced the Portuguese tactician and United to put a timescale on the 30-year-old's absence from action. His continued absence will be a blow to the Red Devils, who were left with limited options in the centre of the pitch for the last three matches.

Paul Pogba was serving a three-match suspension and Michael Carrick is yet to return to the pitch after recovering from a heart condition. The Frenchman is available for United's trip to face Bristol City in the quarter-final clash of the Carabao Cup on Wednesday.

Meanwhile, Fellaini's contract at Old Trafford will run down after the end of the 2017/18 season. He has not yet committed his future to the club and a failure from United to tie him to a new deal before the end of this calendar year will then allow him to negotiate with clubs outside England.

The ESPN report claims United are unlikely to allow Fellaini to leave in the January transfer window, even if it means they have to let him leave for free next summer.

Marouane Fellaini
Marouane Fellaini has not featured since Manchester United's 1-0 win over Brighton in November Getty