Willian has again attracted interest from Barcelona, but Maurizio Sarri has told him to forget about leaving Chelsea in January.
The Chelsea boss thinks it is "impossible" for Willian to leave during the current transfer window, despite reports suggesting he could be part of a swap deal with Barcelona involving the out-of-favour Malcom.
Brazil international Willian was the subject of strong speculation in pre-season, with Barcelona reportedly having offers for him rejected.
In the end, Barca signed Malcom from Bordeaux to bolster their wide options, but the 21-year-old has hardly featured, playing just five times in LaLiga.
Malcom cost Barca €41 million ($A65 million) and his slow start has seen a number of transfer and loan links crop up, with Everton originally thought to be front-runners before Marco Silva denied interest.
The latest rumours suggested Barca tabled an offer for Willian which amounted to €55 million ($A87 million) including Malcom, but Sarri says the Chelsea man simply is not leaving.
When asked if he would be happy for Willian to leave, Sarri told reporters: "Of course not, because as you know a winger for us is very important.
"I think he can do better, because the potential is higher, but he's very important for us, so I think it's impossible to lose him in January.
"I didn't receive anything [about Willian] yesterday [Thursday]. You'll have to ask the club and ask about the market, because if you ask me about the market you put me in trouble, because I don't know."
Elaborating on his desire for Willian to play at a higher level, Sarri wants the winger to show more initiative off the ball and to make himself a bigger threat in the penalty area.
"Usually he wants ball on the feet, so I think he can improve in movements without the ball," Sarri said. "He has the potential to score more."
Alvaro Morata has also been heavily linked with a move away from Chelsea and is expected to depart in one way or another this month, with Sevilla interested in a loan deal and Atletico Madrid said to be considering signing him permanently.
If Morata does leave, Sarri will be insisting on a replacement.
"Of course, if he goes to another club we need a replacement, it's normal," the Italian added.
"At the moment, he is here, so I think he has to think only of playing, because in this moment in Europe there are another 300 players involved in the market and they must only think to next matches or it's impossible to play."