Assistant coach Elston Turner took over in Adelman's place. It is the first time Adelman has missed a game of any kind since taking over the Rockets before the 2007-08 season.
Adelman stayed in the locker room during the game and did not address the media before the game as he usually does. Adelman ranks 11th on the NBA's all-time list for wins.
Turner said the whole coaching staff has been battling a bug.
The Rockets open the season on Tuesday at the Los Angeles Lakers.