"Yes, there was a period about 10 years ago where clubs were given to managers who were players who had just finished playing," said Ferguson.
"I don't think any of them, apart from Bryan Robson, worked. It's not easy going in off a playing field to manage top clubs. Going to start at the bottom is a good way.
"Harry has had different challenges to what I face and I think his affinity with Portsmouth is a strong one. There's something about Portsmouth and Harry that seems a good combination. They are probably made for each other and he's proved that by turning the Newcastle job down.
"That was a big temptation for Harry. Any manager would be tempted to take on a challenge like that, but he turned it down because he recognises where he's best placed."