This is true. Any good manager will have the resources and players to do well at Utd, but there is no one around at the moment, and arguably no one in the history of English football, who has ever been able to achieve the same level of domestic dominance as SAF, particularly when facing such a strong degree of competition.
Until recently Mourinho would probably have been the best bet after he did it at Chelsea. But given how meekly his Real team have surrendered the league this year you have to question whether he could handle the Utd job for more than a couple of seasons.