The American crooner struggled Thursday during a show at London's O2 Arena that had been rescheduled from the previous day.
"After consultation with doctors, he has been advised to reschedule the upcoming dates in order to get the rest he needs to ensure a complete recovery," Usher's representatives said in written statement.
People magazine said Saturday a German tabloid reported Usher allegedly lip-synced two numbers Thursday and his dancing was not up to snuff. The performance earned some boos from the audience.
Usher said in the statement he regrets bombing and having to reschedule his shows.