Sir Alex Ferguson has warned David Moyes the biggest challenge will be the "enormity" of Manchester United when taking over as manager and said his successor will decide if Wayne Rooney should stay at the club.

After Sunday's trip to West Bromwich Albion, Ferguson steps down in his 27th year in charge of United. Asked what Moyes should give the most consideration to when becoming manager, Ferguson said: "I think the enormity of the club – he will soon realise that anyways. The global brand and number of sponsors we have here, he has to fit into that. I don't think that's an issue, though, and the most important thing is the team.

"He's got a good squad of players and he will want to add to that, I'm sure, himself. He will have his own ideas and that's good. He will be fine.

"The priority is the football team. Without the football team, they would not have all the sponsorships. The team is the priority – we all know that here. There is no doubt about that."

Three weeks ago Rooney met Ferguson to verbally ask him to leave United. But Ferguson claimed he does not know why the striker wants the move. "I've no idea. It is not my department. I am no longer interested in that. David is going to deal with that. Quite rightly he wants to deal with that and I'm happy with that," the 71-year-old said.

Ferguson does not know if Moyes could persuade Rooney to stay. "I have not spoken to David about that. I'm sure he will address it when he gets here," he said.

Regarding whether Moyes could lead the club for up to 15 years, he said: "Well, I hope so. I think it is the one club who could do that, you know, by showing great loyalty to their manager."

A year after Manchester City won the title, they are now without a manager following the sacking of Roberto Mancini, and Ferguson believes the decision was wrong. "It is quite amazing. He has won the [FA] Cup, been in the final, second in the league and won the league and it's not good enough? I was surprised but actually no, you can't say you are surprised, not with some owners today. You can't be surprised but I still don't think it was right," he said.

Will Mancini's departure make it easier for Moyes next season? "What happens at Man City won't have any bearing on David.He's got the structure here, he's got the experience, he's had to handle things at Everton that are more difficult probably. I don't think that matters," Ferguson said.

Ferguson added he had talked to Moyes on more than one occasion about taking charge. "Yeah, I have spoken to him a few times," he said. "Obviously he will want to come in quite soon I would imagine when the season finishes and speak to the staff and things like that."