Sager's son, Craig II, revealed his father's illness on his Twitter account on Thursday.
"My Dad's 3-4-week acute leukemia treatment starts [Friday]," Craig Sager II tweeted. "Be thinking of him & let's get him back on the sidelines soon."
Known for wearing flamboyant suits during his broadcasts, the 62-year-old has spent nearly 20 years covering NBA games for TNT and also worked several Olympics for NBC. He has also been part of sister station TBS' coverage of the MLB Playoffs and previously served as studio host for both Atlanta Braves and Hawks telecasts.