The west London club signed the older Hazard brother this summer for a fee thought to be in the region of £32 million, and have now set their sights on bringing his highly-rated sibling to Stamford Bridge.
The 19-year-old, who like his brother plays in a wide midfield role, has made just 11 appearances for the French outfit since breaking through their youth ranks.
"I've known Thorgan since I arrived at Lens,” Sibierski told La Voix du Nord. “Thorgan is a very good player and I want to keep him. There is a year left on his contract. Last week I met him to discuss an extension for three seasons.
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“After this, we are not taking short cuts. We didn't say that we would have to sell Thorgan to make money as soon as he gets into the last year with us. We do not react like that at all! We want to extend it, first.
“I've since learned that Chelsea are interested in his possible arrival. I can understand why a player would want to leave Ligue 2 to go to Chelsea. We're in discussions.
“Now, like in every transfer with a year left on his contract, it must be paid for. There are also compensation fees for training him."
Thorgan Hazard has scored 10 times in 23 appearances for the Belgium Under-19 side.