Ferguson has spent £20millionon David De Gea and Anders Lindegaard, but 1966 World Cup winner Banks is unconvinced that either will ever be as good as Hart, who was picked up by Manchester City for a bargain £600,000 from the Shrews six years ago.
"I am not sure about the two keepers at United, not at all," said Banks to the Daily Express.
"Sir Alex Ferguson could have got a better keeper than either of them.
"I am sure he wishes he had signed Joe, who is the best English goalkeeper by a mile and could become the best in the world. He is now making terrific saves for Manchester City, while these lads at United look as if they can let in important goals.
"If you have a reliable goalkeeper, it makes the job of the back four so much easier. In my time, a manager always built his team on being strong at the back."