Fellaini has been linked with a move to Galatasaray, but Mourinho has told the Turkish giants to forget about signing the Belgium international, per Goal's Jamie Smith: "It's easier for Galatasaray to get me than to get Marouane. Forget it. He's too important to me."
Mourinho rebuffing interest from Galatasaray comes amid reports Fellaini has agreed terms with the club. Tweets from Emre Sarigul of Turkish-Football.com (h/t James Benson of the Daily Star) detailed Galatasaray's approach to Fellaini: "Galatasaray have agreed personal terms with Fellaini. His representatives are well aware his days at Man Utd are numbered. Talks ongoing."
However, Mourinho sounds determined to keep the 29-year-old former Everton man. Yet despite the United manager's words, Fellaini is likely to find playing time hard to come by at Old Trafford this season, at least as a member of the starting XI.
United can already call on Basque pass-master Ander Herrera and France international Paul Pogba in midfield. Mourinho has also added to the ranks by acquiring Nemanja Matic from Chelsea for £40 million, per BBC Sport.
The arrival of Matic could have Fellaini thinking about the exit door in Manchester. He could push for a move in January if he doesn't get one this summer, according to the Daily Mail (h/t Metro).
Of course, Fellaini wasn't a regular starter last season, even though he made 28 appearances in the league, per WhoScored.com. Fellaini also chipped in with three goals and two assists across all competitions.
More than his numbers, the veteran added the physical qualities Mourinho often likes his teams to possess. His versatility also makes Fellaini valuable to his manager.
Mourinho can bring him off the bench to use his height defending set pieces and long balls to help protect a lead. Or he can be used in advanced midfield areas to help break up play against teams who play out from the back, as well as offering United a target when they want to adopt a direct style.
Mourinho used Fellaini as part of the latter tactics to help beat Ajax and win the 2017 UEFA Europa League final.
It's obvious Fellaini still has a place in his manager's plans, even after the arrival of Matic. However, it won't be long before Mourinho needs to think about making United younger in the middle, with Fellaini nearly 30, while Michael Carrick is 36.
Wanting to get younger led United to Anderlecht's Leander Dendoncker. The Red Devils made an approach, but are not likely to sign the player during this window, per Ciaran Kelly of the Manchester Evening News: "having scouted Dendoncker at length, MEN Sport understands that United made an enquiry earlier this summer and touched base with his representatives."
Kelly also noted how "Anderlecht value the 6'2" powerhouse somewhere between £22 million and £31 million."
Interest in a burgeoning talent such as Dendoncker shows Mourinho has one eye on the future when it comes to his options in midfield. The physical description of Dendoncker also shows the United boss will continue prioritising strength in the middle.
It's also why Fellaini is likely to remain a United player, at least for another season.