Tests last week revealed that Sager's leukemia is back, Craig Sager Jr. announced Sunday on Twitter.
"Sr.'s leukemia returned last week and he is undergoing treatment that will be much like this time last year when he got rounds of chemo and prepared his body for a bone-marrow transplant," the younger Sager wrote, noting that his father also has been battling the flu.
Craig Sager received a standing ovation from the crowd at the United Center in Chicago when he returned to the sideline March 5 for a game between the Bulls and Oklahoma City Thunder. Sager was diagnosed with acute myeloid leukemia in April 2014 and received a bone-marrow transplant from Craig Sager Jr.
Craig Sager was embarking on a limited schedule since his return, but announced on March 18 that he would not be reporting on the NCAA tournament since his doctors wanted him to undergo more testing.
"Physically, I feel great," Sager wrote in a post on a Turner Sports Twitter account. "I'd like to thank everyone for the overwhelming support throughout this journey and look forward to being back very soon."
Sager, 63, is perhaps best known for his eccentric and colorful clothing style, and has worked as a sports television reporter since the early 1970s. In recent years, he has worked primarily as an NBA sideline reporter for TNT.