World heavyweight champion Vitali Klitschko‘s title defense against American Hasim Rahman has been postponed until at least September, ESPN reported Monday.

The Ukrainian, due to fight Rahman on July 23, is to have back surgery.

"It‘s something to do with his back that is causing a problem with a nerve in his leg," adviser Shelly Finkel told ESPN.

"On Wednesday they‘re going to do a procedure that should alleviate it. He‘ll probably fight in September or October but we hope September."

Klitschko was originally scheduled to fight former champion Rahman this month but pulled out because of a thigh injury. The 33-year-old Klitschko retained his World Boxing Council crown when he stopped Britain‘s Danny Williams in the eighth round at the Mandalay Bay Resort and Casino in Las Vegas in December.