The 30-year-old, whose Old Trafford future continues to hang in the balance, has only managed to make one start since September due to a continued problem with his knee that surfaced whilst on international duty.
And he was forced off just eight minutes after coming on to replace Paul Pogba, with many initially thinking that Mourinho had taken him off for obscure tactical reasons as his frustration on the touchline mounted.
But the 54-year-old, speaking after the game, confirmed that Fellaini came off with the same knee problem that has plagued the majority of his season.
Mou on Fellaini injury: "It's the same [as the last one]. It's the lateral ligament." #MUFC
— Kris Voakes (@krisvoakes) January 31, 2018
This is starting to become a real issue. That excellent start to the season, combined with Mourinho’s praise of the midfielder, led to many thinking Fellaini could play a pivotal role in midfield, but ever since injuring his knee ligaments on a boggled pitch in Bosnia and Herzegovina he has been almost invisible.
Mourinho is sizing up a midfielder to replace Michael Carrick ahead of the summer, but Fellaini’s persistent injury woes may necessitate an early search for another – perhaps more attacking – man in the middle over the next few months, regardless of whether the Belgian decides to stay or not.