Shingo Katayama of Japan has withdrawn from the U.S. Open because of an injury to his upper back.
Katayama is No. 39 in the world ranking, and finished fourth at the Masters in April, two shots out of a playoff. He is the fourth player to withdraw from the U.S. Open, which starts Thursday at Bethpage Black.
He was replaced in the field by 19-year-old David Erdy of Indiana, the first alternate from sectional qualifying in Dayton, Ohio.
Erdy is the 16th amateur in the field, the most amateurs in more than a decade.
Scott Lewis, another amateur, previously replaced Dudley Hart, while Clinton Jenson replaced former Masters champion Trevor Immelman. Robert Karlsson, who won the Order of Merit last year on the European Tour, had to withdraw last week with an eye infection. He was replaced by Andrew Svoboda.
(This version CORRECTS to 16 amateurs)