Howell, who won last year's tournament by five strokes at 17 under, hasn't recovered from back and hip problems which forced him to pull out of his last two tournaments in the United States.
"I would love to have gone out and defended it, but if you are not fit you are not fit," he said. "It was always touch and go, but in the end I felt I couldn't risk it. At the end of the day, it was a commonsense decision."
Two of the tournament's other big names look set to play after recovering from injuries.
Darren Clarke, who missed last week's Irish Open with a hamstring injury, is expected to tee off in Thursday's opening round. Justin Rose played 18 holes Monday after resting his bad back.
Five of the world's top 10 - Ernie Els, Vijay Singh, Henrik Stenson, Retief Goosen and Luke Donald - are in the field.
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