The Frenchman took to social media on Thursday to announce that Lucas had died.
“It is with great emotion that I inform you that Lucas has gone to join the stars,” he wrote. “Very difficult to find the words but it is necessary to pay homage to this boy filled with love, sympathy and above all courage who taught me so much and with whom I nurtured a strong bond.
“I will try to make you proud because you are the real hero.
“You who deserve all this praise for having fought your disease for more than 10 months without ever complaining.
“I am extremely proud to have been able to be your friend until your last breaths and believe I will be forever.
“May God grant you the way to Heaven my little angel.
“I love you. Kylian.”
Les Petites Bosses, an organization that provides moral and financial support for sick and disabled children, also confirmed the news:
“Our little Lucas left this morning to join the stars. This little angel fought like a champion against disease, surrounded by his family.
“At only 8 years old, he had an extraordinary and inspiring personality. A model of strength and courage for us.”
After opening the scoring in last Friday’s 4-0 win over Nimes – in which he found the net twice – Mbappe lifted his shirt to reveal a message dedicated to the youngster which read: “Courage Lucas, I’m with you.”
He also paid tribute to Lucas whilst speaking to French broadcaster Telefoot at halftime in the game, saying: “I would like to make a little dedication to Lucas, a friend of mine. We are one of the privileged, some are not so lucky.”
Lucas was an enormous PSG fan, particularly of 21-year-old FIFA World Cup-winner Mbappe.
The forward raised money for Les Petites Bosses by auctioning off one of his PSG jerseys.
In August, PSG invited him to a screening of the UEFA Champions League final in Paris at the Parc des Princes when they took on Bayern Munich, but he was unable to attend.