Ronaldo will be welcome at Napoli, but does Napoli need that at the moment? Playing and performing better than they do now is actually unimaginable, and with a player like Ronaldo in a team, the strategy will also have to be partly adjusted.
Napoli do not need Ronaldo or any player in January, they are perfect the way they are and the best thing they can do for themselves is to continue in this way without bringing any player in, and surely not a great player like Ronaldo, because it is hard to manage big players, and the coach may run into problems as he tries to manage Ronaldo, just the way Erik is having many problems managing Ronaldo. I have just seen a statement from Napoli that they would not be buying any player in January, so that has to be the end of this rumor, Ronaldo may not leave United in January, not because he does not want to leave, but because there is nobody to buy him.
This is just a rumor that is being exaggerated because regardless, there has been no statement from both parties. But indeed until now there have been some rumors that indeed say that Manchester United are now trying to offer Ronaldo to several clubs and maybe if Napoli really want they can join but of course this will disturb their stability I think and I don't think Ronaldo will re-enter Serie A again.