Vitali Klitschko`s comeback has been put on hold after the former WBC heavyweight champion injured his back in training, according to SkySports.
The German-based Ukrainian, who retired in 2004 claiming his body was no longer up to the rigours of training, was due to fight Jameel McCline on September 22.
However, the 36-year-old suffered back pain at his camp in Austria and was forced to undergo emergency surgery in Innsbruck on Saturday.
Klitschko, whose brother Wladimir is currently the IBF champion, lost to undisputed king Lennox Lewis on cuts in 2003, but won the vacant WBC belt against Corrie Sanders less than a year later - defending it once against Briton Danny Williams.
The muddled state of the division was the apparent motive for `Dr Ironfist`s` comeback, with the Klitschkos still harbouring the ambition of simultaneously holding world titles.
Klitschko has been told he must rest for six weeks, but he admitted on Monday: "I feel good now and I have no pain. I have the feeling I could start training again in a day or two but the doctor told me not to overdo it."