Manchester United midfield Marouane Fellaini's knee injury continues to mystify his club's medical staff, as the Belgian international pulled out of the Manchester derby with just 24 hours notice.
The 30-year-old missed much of October with strained ligaments, but succumbed to injury again against Brighton at the end of November.
As reported by ESPN, sources at Old Trafford have claimed they're perplexed by Fellaini's injury, as scans have failed to determine the cause of the former-Everton star's pain complains.
Fellaini has just six months left on his contract, but United are believed to be prepared to block any approaches for the midfielder in January.
A number of sides across Europe are thought to be keen on luring the player away from the Premier League, while a move to the lucrative Chinese Super League has also been suggested.
United boss José Mourinho is believed to have been deeply frustrated by the loss of Fellaini for the Manchester derby, with Paul Pogba's suspension leaving their midfield light.
Fellaini has become renowned as Mourinho's not-so-secret weapon - deployed against the Red Devils' tougher opponents as a form of battering ram.
Standing at 6ft 4', the former Standard Liège man has the height and strength to play the role of a brawler when required - unsettling opposition defenders with his ability to cause chaos with this physical attributes.
United face an uphill task if they are to win their first Premier League title since 2013. They currently sit 11 points behind their fierce rivals Manchester City - who are top of the league after winning 17 of their opening 18 matches.
City edged United 2-1 in the Manchester derby, claiming the precious bragging rights at Old Trafford.