I don't know what plans Chelsea have for replacing Lukaku but Christiano Ronaldo moving to Chelsea seems very unlikely. He wouldn't do that to Manchester United. Lewandoski might be open to a move to Chelsea and with his price tag Chelsea are in a better position to get him than Barcelona at the moment and I think he will be open to the idea of moving to the English premier league.
You are right, if Ronaldo moved it would probably be to another league, we saw at the beginning of his move to Manchester United, that the first one closer to this Portuguese star was Manchester City, even the news has spread so widely. But those behind Ronaldo didn't let that happen and did something so he joined Manchester United again. I think it will happen again if another team in the Premier League wants Ronaldo.
I don’t see Christiano Ronaldo switching sides to Chelsea. He’s a responsible player and knows what rivalry means. I am aware that Chelsea Owner met with Ronaldo's Agent to discuss about a possible transfer but it is not reliable. I think people might have thought that his frustration over Manchester United’s lack of activity in the transfer market while other top-six Premier League clubs have continued to bolster their squads is the reason why he wants to leave. Ronaldo is an important part of Erik Ten Hag's plan so there’s no point if he leaves. Chelsea will have to look elsewhere for a new striker to replace Lukaku as Lewandoski has only Barcelona on his mind at the moment.
There are several clubs that are rumored to be the goal of Cristiano Ronaldo next season. AS Roma, Bayern Munich, Inter Miami, Chelsea, Sporting and Inter Milan. The news circulated considering that Manchester United will not appear in the Champions League next season and Ronaldo is still very well-known for being reported by the media.
Erik ten Hag has already said that he will be with Ronaldo next season to rebuild Manchester United.