Day had an MRI in Miami on Wednesday that was negative but the doctor prescribed rest if he was unable to play.
"My left thumb was bothering me at the WGC-Accenture Match Play Championship and the pain continued here this week," Day said in a statement. "My plan is to rest for a couple of weeks."
Day, who clinched the biggest title of his career two weeks ago at the WGC-Accenture Match Play Championship, arrived at Doral as one a favourite to win the $9 million WGC-Cadillac.
Partnered with Northern Irishman Rory McIlroy and 2009 Doral winner Phil Mickelson, Day pulled out just prior to teeing off and will now focus on getting fit for the April 10-13 Masters, the year's first major.
- Sports & Recreation