Paul Casey will be replaced at the US Open by 14-year-old Andy Zhang after the former world number three withdrew through injury.
Casey dislocated his shoulder in a snowboarding accident on Christmas Eve, and does not yet feel fit enough to contend with the rigours of a major.
"You're always disappointed to miss time due to injury, but it's especially true when you have to withdraw from the US Open," said Casey.
"I've been trying to play my way back and I feel like I probably came back too soon originally.
"It's affecting my ability right now to be ready for the test of a major. I do feel like I'm very nearly ready, but not quite ready for this week."
China-born Zhang's appearance in San Francisco will make him the youngest player at a US Open since the Second World War.
