Rio Ferdinand cannot imagine Manchester United without Sir Alex Ferguson, and says the Red Devils manager's job is 'the hardest in the world'.[LNB] The legendary Scot is entering his 25th year as the main man at Old Trafford, and during that time he has returned the club to the pinnacle of English and European football.[LNB]He has landed countless honours during his reign, with the 2010/11 Premier League title seeing United overtake arch-rivals Liverpool as the most successful side in English top-flight history.[LNB]Much of Ferguson's success can be put down to his ability to identify new recruits and bring the best out of them.[LNB]Ferdinand is undoubtedly one of his best pieces of business down the years, with the centre-half providing full value for money despite boasting a £30million price tag.[LNB]The England international has spent almost a decade marshalling the Red Devils' back four and he admits that life at Old Trafford would not be the same without Ferguson leading the club forward.[LNB]Speculation as to how much longer he can go on for continues to rage, but Ferdinand hopes it will be long after he finishes playing before Ferguson finally decides to slip into retirement.[LNB]"I don't see the club without the boss," he told the Jakarta Globe.[LNB]"I like how the manager works and we enjoy being with him. [LNB]"I hope he can stay longer than all of us."[LNB]Numerous names have been offered up as potential successors to Ferguson, but Ferdinand is not convinced that there is anybody working in management at the moment that could fill his shoes.[LNB]He added: "It's difficult to imagine (United without Ferguson). I think being the manager of Manchester United is the hardest job in the world.[LNB]"You could say this man or that man, but when they get there they may crumble or fail. It's hard to say."