Calls to the foundation and doctors at the Osborne Head and Neck Institute were not immediately returned Saturday.
In photos and videos posted on Instagram, Mutombo greeted the boy when he arrived at the airport with his father and accompanied him to a medical appointment.
Matadi was born with a cleft lip and a tumor on one side of his cheeks that grew into the size of a balloon.
“We are devastated by the loss of Matadi and our heart goes out to his father, his mother and the rest of his family, and all his old and new friends,” the statement said. “We are comforted by the knowledge that a whole ‘village’ adopted Matadi and joined forces to offer him treatment that was not available in the Democratic Republic of Congo and which would have given him the opportunity to have a new and different life.”