Jose Mourinho has expressed his concern that Marouane Fellaini could leave the club for nothing in summer 2018.
The Belgian's contract expires next summer, with Goal having previously revealed that he has turned down an offer of new terms to remain at Old Trafford.
Fellaini 21/10 to net vs. Brighton
As revealed by Goal, Fellaini's agent had approached Serie A champions Juventus over a possible more for the player, although the Italian giants are not interested in signing him.
And Mourinho insists that the matter is out of his hands now and that it is up to the board to negotiate with the former Everton midfielder over a new deal.
He told reporters: "Yeah, I am [concerned Fellaini could leave]. It is a discussion between the player and the board.
"Discussions are under [the board's] control. So I just wait, wishing that they will have an understanding but it's out of my control."
Fellaini has featured in 11 games in all competitions for United in the 2017-18 campaign and has emerged as a key player under Mourinho's management.
The Portuguese has previously expressed his admiration for the Belgian, commenting that he feels "weaker" without the midfielder in his team.
"He's an important player for me - a lot more important than you can imagine," he said back in September.
"I feel weaker without Fellaini in my squad, doesn't matter if it's on the pitch or on the bench. If his conditions are improved, he will be selected, I need him."