He said: "It would have been nice to have had another defender. It is something we are exploring but nothing is jumping out in front of us."
At today's press conference where his two latest signings were unveiled, Ferguson also revealed he had been tracking Kleberson for around six months - and Ronaldo for nearly a year.
But the 18-year-old's display in last week's 3-1 friendly defeat by Sporting Lisbon convinced United to move in.
He said: "We had a tough night against them. He has enormous talent there and he wants to play. He's not fazed by anything.
"As with every young player, there will be times when you have to leave him out and rest him."
Ronaldo, who looks set to make his debut in tonight's friendly at Stoke, will wear United's famous No7 shirt. The number was previously worn by, among others, David Beckham, Bryan Robson and Eric Cantona.
Meanwhile, Ferguson has branded Juan Sebastian Veron a "disgrace" for claiming he was misled over his £15m move to Chelsea.
The Argentina midfielder accused his former manager of not being straight with him during the on-off transfer which was finally resolved last week.
But Ferguson today said: "Veron knew all along what was happening, so to say I was two-faced about it is a disgrace."
