Lennox Lewis says Wladimir Klitschko must become the undisputed heavyweight champion of the world before he can be considered a boxing great.
Klitschko - who faces Tyson Fury in Dusseldorf on Saturday - currently holds the WBA, IBF, IBO and WBO belts but is missing the WBC title - currently held by Deontay Wilder.
The division has not been unified since Lewis beat Evander Holyfield in 1999.
"He must have all the championship belts," Lewis told Welt am Sonntag. "I had them all.
"Although Klitschko has the championship belt of four world organisations he lacks the decisive one.
"In addition to the IBF and WBA belt you have to especially have the WBC belt. That is the most important.
"No matter what, he must fight [Wilder] to get this belt. If he does [win] he should say goodbye to boxing, it would be the perfect ending to a brilliant career."