The Englishman has been replaced in the 156-man field by American Jason Kokrak.
Westwood, 45, is perhaps the best player of his generation never to win a major title.
Three times he has finished runner-up and has six other third placings.
The PGA Championship, the final major of the season, starts at Bellerive in St. Louis on Thursday.
(Reporting by Andrew Both in Cary, North Carolina; Editing by Ian Ransom)
