The team made the announcement minutes before the start of Wednesday night's game. It was unclear when Howard was injured. He appeared to be fine at the morning shootaround.
Howard is averaging 18.8 points, 11.3 rebounds and 2.3 blocks per game in his second season with Houston. Rockets rookie Tarik Black started in his place.
Howard spurned the Lakers in 2013 after one season with them to take roughly $30 million less as a free agent in Houston.
He had 13 points and 11 rebounds in a 108-90 victory over Los Angeles on opening night, when Howard got into a shouting match with ex-teammate Kobe Bryant. They received matching technical fouls when Howard elbowed Bryant in the face after grabbing a rebound in the fourth quarter.