The Belgian picked up what was labelled as a knee ligament sprain during a clash with Bosnia and Herzegovina back in early October and was out for over a month.
He returned from the bench at Stamford Bridge and started against FC Basel in November but has not featured since a substitute appearance against Brighton and Hove Albion.
And according to ESPN‘s Rob Dawson, the club’s medical team have put the midfielder’s knee through a series of scans but still haven’t unearthed the root of the problem, making any return date impossible to predict.
The fact that United, even with the very best medical resources at their disposal, cannot decipher exactly what is wrong with Fellaini’s knee is worrying to say the least.
To have absolutely no idea at this level is extremely rare – unheard of, even – and it could mean that Fellaini’s injury is far more deeply ingrained and complex than first thought.
Supporters will just hope to receive some clarity on a player deemed hugely important by Mourinho sooner rather than later.