The 27-year-old has scored 10 goals and registered one assist in 18 appearances for the Blues this season, but he has only scored once in 11 Premier League matches, with the Frenchman not a part of Maresca's first-choice XI in the league.
Nkunku has only started once in England's top flight this term - his other 10 league appearances coming off the bench - with his best football for the club being played in the Conference League and EFL Cup this term.
There have been suggestions that the Frenchman is unhappy with his current role in London, with Manchester United believed to be keeping a close eye on his situation, as the Red Devils look to boost their forward options.
Man United are thought to be weighing up a January move, but Maresca has ruled out a mid-season departure for Nkunku and insisted that he is not aware of any unhappiness from the player's side.
Maresca rules out January departure for Nkunku amid Man United talk
"Not at all. I spoke with Christo days ago - he did not say he is not happy," Maresca told reporters at a press conference when quizzed on the recent speculation.
"Absolutely [he is not for sale], yes. The problem with Christo, not the problem but the same issue we have mentioned many times; if you play with Cole (Palmer), Joao (Felix), Christo, Jadon (Sancho) or Noni (Madueke) - who is defending? Their skill is not defending. The last game, we tried with Joao and Cole together.
"Probably the best moment for Christo with us has been in pre-season, playing in his position. Since we started, he is playing as a nine for us. Against City, he played as a winger; knowing this is not his best position. His best position is as a 10 but Cole is playing there. We need balance."
Man United are now playing a 3-4-3 system under new head coach Ruben Amorim, with Nkunku potentially operating as one of the number 10s, but it will seemingly be very difficult for the Red Devils to acquire him in January.
Which other strikers are being linked with Man United?
Man United are highly unlikely to sign a big-name striker in January due to the difficult nature of the market, but there has been suggestions that a player could arrive on loan to boost a struggling department.
Paris Saint-Germain's Randal Kolo Muani is thought to be a target for January, and a loan deal for the second half of the 2024-25 campaign could suit all parties, with the attacker currently struggling in the French capital.
Man United are set to move for a long-term forward next summer, meanwhile, with Sporting Lisbon's Viktor Gyokeres said to be their leading target, but Inter Milan's Lautaro Martinez is also being linked with a switch to Old Trafford.
Rasmus Hojlund is expected to remain with Man United, but the arrival of a new forward would likely lead to the departure of Joshua Zirkzee, who has only scored once for the Red Devils since arriving from Bologna over the summer.