Fellaini's current contract expires at the end of the season and potentially he could sign a pre-contract agreement with another side in January.
That's according to the Daily Star, who claim the Belgium worldwide is unsatisfied with the new contract he's been offered at Old Trafford.
Manchester United fear Marouane Fellaini could leave the club for free next summer with contract talks remaining at an impasse. It now seems as though Fellaini will move clubs in the near future, as United look to recoup money for the Belgian rather than let his contract run down.
However, the report also claims that Arsenal are interested in the Belgian alongside Inter Milan and Besiktas.
Fellaini has emerged as a key part of Mourinho's team, after he endured a hard start when signing for David Moyes for £27.5m in the summer of 2013.
But Moyes successor Louis Van Gaal and Mourinho both valued him for his aerial power and ability to unsettle defences, often sending him on as a substitute to change the course of a game.
Fellaini though is rarely a starter when Mourinho can name his first choice starting line-up - he's made only three starts this season.
However, since then, Fellaini has become a more popular figure.