Day, who won last month’s WGC-Accenture Match Play Championship, said in a statement that his thumb began bothering him at the Match Play and the pain continued this week in South Florida.
Day had a MRI on Wednesday in Miami and the results showed no damage.
“The doctor prescribed rest if I was unable to play today. My plan is to rest for a couple of weeks,” said Day, who was not scheduled to play next week’s event in Tampa, Fla.
There are no alternates in the World Golf Championship and Day was not replaced in the field.