Manchester United boss David Moyes has warned the transition from "The Ferguson Era" is going to be tough and could take some time.

Focus sharpened on Moyes after United were thrashed 4-1 by Manchester City on Sunday in his first derby as manager at Old Trafford.

But the Scot says the six-year deal he was handed when he took over from Sir Alex Ferguson showed those in charge of the club knew it was going to be a lengthy process to mould the side under a new manager with different ideas.

"I don't think it was ever going to be the case this was going to be easy and smooth after Sir Alex," said Moyes. "There was always going to be days like this and there might well be more days like this. I hope not, but there will be because that is football.

"It was always going to be tough following such a great manager with a great team and I think people with real football knowledge will know there are probably some changes to be made."

Moyes added: "It is not going to be made in one fell swoop. It is going to be done in time. That is why the Manchester United board realized the job that needed to be done was a long-term one.

"I thought I was (prepared) but obviously when you come here then I realize maybe it wasn't. It is a club on its own. It is a fantastic place to work with great players. But we have to do well enough to make sure people on the outside think the work we are doing is the right stuff."