The Red Devils have endured a mixed 2017-18 Premier League campaign so far despite a positive start to the season.
The 20-time English champions managed to keep pace with Manchester City in the opening seven Premier League games.
However, a disappointing return of four points from four games in October left Manchester United adrift of the leaders.
The Red Devils lost 2-1 to Manchester City in the derby before three successive stalemates with Southampton, Leicester City and Burnley.
Manchester United are 16 points behind runaway leaders Manchester City to leave Mourinho’s men with a lot of ground to make up.
BBC Sport reporter Stone explained what would need to happen for Manchester United to be active in the January transfer window.
“If Man United get the chance to sign a player for the right fee who they are already interested in then that is what they will try and do,” Stone told BBC Radio 5 Live.
“I don’t see Man United waking up on 1 January and thinking, who are we going to sign now?
“It has to be more long-term than that.”
Manchester United signed Romelu Lukaku in a £75m deal from Everton in the summer transfer window.
The Red Devils recruited Nemanja Matic in a £40m transfer from defending champions Chelsea.
Victor Lindelof move to Benfica in a £30m switch from Portuguese side Benfica.
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