The Belgium international is into the final two years of his contract at Goodison Park and could be sold this summer if a big enough offer is tabled.
Reports in the British press suggest that Chelsea boss Antonio Conte is still deliberating over whether to sign Lukaku, holding off making a decision until he has held talks with Costa later this week.
Costa has welcomed the prospect of Lukaku, the scorer of 25 goals in the Premier League last season, joining him at Stamford Bridge, despite the prospect of being pushed closer to the exit if a transfer goes through.
Asked by Belgian paper Het Nieuwsblad if he could partner Lukaku next term, he said: "I do not know, that's a decision for the club. Of course Lukaku is a very big player who has played an impressive season.
"I hope he will ever have the possibility to sign at Chelsea because Chelsea is a unique club."