Mo Farah has to break the 5,000 or 10,000 metres world record to be considered an all-time great, according to Kenenisa Bekele.
Farah heads into this summer's Rio Olympics as the defending champion over both distances, while he has also won five world titles.
Another golden double in August, which would be his fourth in a row at a global outdoor championships, would see him eclipse Bekele's medal haul, the Ethiopian having three Olympic titles and five world outdoor crowns to his name.
Yet Bekele, who races over 10km in the Great Manchester Run on Sunday, has also held the 5,000m and 10,000m world records - 12 minutes 37.35 seconds and 26mins 17.53secs respectively - for more than 10 years.
Farah's personal bests for both distances are some way short of those marks at 12:53.11 and 26:46.57.
Asked if Farah had to take one of those records from him to be considered an all-time great, Bekele said: "Of course. He has achieved (a lot) with Olympics and World Championships, what he needs to take my crown (is to) break those records.
"People all the time compare, but we have to compare everything."
Farah holds the indoor two-mile world record, but such an achievement is not in the same league as Bekele's major long-distance marks.
And there will, the Ethiopian believes, be something missing from the Briton's CV if he retires without a significant world record.
"He will feel something," he said.
"For those events the highest level is the record. If someone is not beating that record, you have to limit them (in the order of great runners). If you don't have that capacity to do that, and if you don't try, something will be missing."
Farah has yet to start his outdoor campaign - he races at the Birmingham Diamond League on June 5 - and Bekele reckons it is too early yet to label him the favourite for Rio.
He said: "Of course he will have a big chance to win both golds, but we have to see if any newcomers break through. But it will not be easy for Mo."
Meanwhile, on Saturday 42-year-old mother of two Jo Pavey will aim to book her place in Rio when she races over 10,000m in Highgate.
The Highgate Harriers Night of the 10,000m PBs race will act as the trial for the Olympics, with Pavey, the European champion from 2014, needing to finish in the top two and run the qualifying time of 32:15.00 to guarantee her spot on the plane.