By the way we don't even know when he will return. This might be a hindrance for Chelsea if they want to let him go in January already.
I for one do not also know when N'golo Kante will return to the field for Chelsea, prolly after the WC, but i don't see that as being too big a problem for Chelsea, footballers are bound to injure, it is part of the game, and in the period in which they are on the treatment table, the club basically have to wait for them to get well, and that is what i expect Chelsea to do in N'golo's situation and not to sell him off, i know it is not concrete news yet that Chelsea are willing to sell him in January, and tbh i don't expect it to be true.
Having said that, i know of Chelsea's policy of restricting players above 30 years of age to just a year's contract extension, and i think N'golo is going to finish the season with Chelsea, and should get the offer of a year extension, of which he could then leave if he is not comfortable with that. He has served Chelsea well, and there is neither any serious drop in his form, so i expect the club to be patient with the player, except of course if the decision to leave will be coming from his own camp, of which the club would willingly accept.