It was announced at Manchester's Verizon Wireless Arena that McPhee, the runner-up to winner Taylor Hicks on this year's American Idol, would be missing both Wednesday's and Thursday's performances.
Her doctor, Sean Nassari, ordered "total voice rest," the Associated Press reports.
In a statement, McPhee, 22, said that, while she was disappointed not to perform, "I need to follow my doctor's orders to recuperate and reunite with the rest of the idols."
Adding that her doctor had told her she needed to remain silent, she said, "That's the most difficult part, not talking!"
McPhee, who has been singing since she was 2, lives in Sherman Oaks, Calif., and has said she hopes eventually to act in movies as well as musical theater. Her versions of "Over the Rainbow" and "My Destiny" were released on June 27.