The search for a new boss is underway with Ferguson announcing his retirement on Wednesday, and Robson has admitted that the Scot's successor will have a tough task on his hands.
"For me you have to say he has been the greatest club manager ever in the world. He has rebuilt four or five squads to be successful, and look what he did at Aberdeen as well. Because of that record, he has to go down as the best," he told Sky Sports News.
Robson spent 13 years at United.