The team said in a statement Thursday the 60-year-old executive received immediate medical attention Tuesday night in Milwaukee, where the Celtics were facing the Bucks in the second round of the playoffs. Ainge is returning to Boston.
He also had a mild heart attack in 2009.
Ainge was a baseball and basketball star at BYU who played parts of three seasons with the Toronto Blue Jays before settling into a 14-year career in the NBA. He won two championships with the Celtics in the Larry Bird era.
Ainge coached the Phoenix Suns for three-plus seasons and took over the Celtics basketball operations in 2003.