The Belgium international has been a key player for the west London club that saw the Blues sit at the top of the table under Antonio Conte's guidance. He, along with Diego Costa and Pedro, have formed a partnership to lead Chelsea's attack this season.
Hazard signed a new deal at Chelsea in February 2015 said to be worth £200,000-a-week ($247,980). His club form has seen him attract interest from top clubs across Europe, including Real Madrid.
According to the Spanish newspaper Marca, Chelsea are under obligation to negotiate a transfer if the 11-time European champions make an approach for Hazard's services due to the agreement reached between the player and the Blues director Marina Granovskaia last year.
Calderon, who was the chief at Real believes that the La Liga giants will have to pay a world-record fee of £100m ($124m) in bringing Hazard to Spain. He stressed the only way Chelsea can retain their star player beyond this summer is if the attacker decides against leaving the Premier League leaders.
"Chelsea aren't a selling club, so it wouldn't be easy, but as always it would depend on the wishes of the player. If he wants to leave, he will leave - that's always the point," Calderon told talkSPORT.
"Chelsea will ask for a lot of money, I think that's what the situation will be at the end of the season.
"Chelsea knows quite well how much has been paid in the market over the last few years, with Ronaldo, Bale and Pogba - £100million is now the bar.
"That bar has been set by those transfers, so I think that's how much a club like Chelsea would be asking for and the economy at Real Madrid is healthy - I don't think that wouldn't be a problem for them.
"But it's all about the player. The player must say, 'I want to leave and go to Real Madrid', and that will be the moment they start negotiating."