The 30-year-old, who plays for Italian club Benetton Treviso, has been sidelined for the past few months and had an operation in August to remove an abnormal growth.
The No 8 had blood tests last week and the club has announced the need for him to now undergo chemotherapy in the next few days.
Manu said in a statement that he was determined to win his battle against cancer and was grateful for the support he has received. "Thanks to my club, from management to teammates, I appreciate all your support," Manu said.
