Mickelson will not be playing because of a scheduled family vacation, while Casey is not in the field as he recovers from a shoulder injury. Casey was the runner-up in 2009 and '10.
Ernie Els and George Coetzee replaced Mickelson and Casey.
World No. 1 Luke Donald returns to defend his title, and will be challenged by Lee Westwood, Rory McIlroy and Martin Kaymer -- Nos. 2-4 in the world, respectively. Donald defeated Kaymer 3 & 2 last year to earn the victory.
Tiger Woods, a three-time champion, is also in the field of 64.
Seeding for the championship will be based on the official world golf rankings as of the Monday of championship week -- February 20 -- rather than the rankings at the time the field was finalized.