The players spent last season on loan at Sunderland and Wigan respectively.
"Welbeck's an England international, an exceptional talent. Cleverley will play for England too," said Ferguson.
The manager insisted that both players will not be loaned out this season and will benefit the side as the club aims for its 20th league title.
"We are happy both of them will stay with us," he added. "Because of the experience they have had, keeping them now benefits us."
United begin the defence of their Premier League crown on Sunday 14 August away at West Bromwich Albion.