Eight-time NBA All-Star Dikembe Mutombo is undergoing treatment for a brain tumour.
The 56-year-old DR Congo-born centre spent 18 years playing in the league, including spells with Denver Nuggets, Atlanta Hawks and Houston Rockets.
The Hawks retired his number 55 shirt in 2015 – the same year Mutombo was inducted into the sport’s Hall of Fame – and the Nuggets followed suit the following year.
“He is receiving the best possible care from a collaborative team of specialists in Atlanta and is in great spirits as he begins treatment,” said a statement from the NBA.
“Dikembe and his family ask for privacy at this time so they can focus on his care. They are grateful for your prayers and good wishes.”
Standing 7ft 2in (2.18m) tall, Mutombo was discovered when he was studying at Georgetown University.
He became the first round pick of the Nuggets in the 1991 NBA draft, going fourth overall.
Known for his defensive game, he led the NBA in blocked shots for five consecutive seasons and blocks per game for a record three consecutive seasons.
He also had spells at the Philadelphia 76ers, the New Jersey Nets and the New York Knicks, and was second on the NBA’s career blocked shots list when he retired in 2001.
As well as being a global ambassador for the NBA, Mutombo has carried out humanitarian work since leaving the professional game which has included funding a new hospital in DR Congo’s capital Kinshasa.