Bolton's Cahill was one the most sought after players this summer, with Arsenal and Spurs just two of several clubs thought to have registered an interest in signing the player.
The England centre-half remained at the Reebok Stadium, however, with neither of the two London clubs willing to match the Trotters' reported £20million price tag.
Redknapp tried to sign Ferdinand two years ago but failed when the former England captain was a regular fixture in the United defence.
Ferdinand's injury problems mean he has played a limited part in the champions' season so far, and his contract is due to expire this summer, but Redknapp has played down suggestions he will bid for either of the duo when the transfer window opens.
"I haven't even thought about Rio to be honest," Redknapp said.
"I signed him as a kid. I love Rio as a boy and a player, but it's not something we have ever enquired about.
"It's going to be difficult for him to leave Man United isn't it?"
The signing of Cahill would have been a big coup for Spurs, especially given that Arsenal were interested in the player, but Redknapp's defence has performed well without him.
Even long-term injuries to William Gallas and Michael Dawson have not been felt by the squad thanks to the strong performances of Younes Kaboul and fit-again captain Ledley King.
That has left Redknapp feeling he does not need to add to his defence in January, even if Cahill becomes available again.
"I'm not looking to strengthen in defence, really. We are strong in defence. We have Dawson, King, Gallas, Kaboul and (Sebastien) Bassong," said Redknapp, who takes his team to face Newcastle at St James' Park on Sunday.
"I went for Cahill because at the time I couldn't see Ledley being fit and because William Gallas had a calf strain."