Ferdinand signed a one-year deal with QPR on Thursday after the 35-year-old was allowed to leave United at the end of last season.
Giggs had been in interim charge for the final four games of last term before being named as No.2 to Louis van Gaal following the Dutchman’s appointment as David Moyes’ successor.
Redknapp discussed his move for Ferdinand with Giggs and has revealed that the former England captain could have been considered for a key role with United in the coming season.
“Rio is a top-class guy – anyone who has watched the TV over the World Cup would see what a world-class act he is, on and off the field,” said Redknapp."He’s a great role model. I signed him as 14-year-old, so I know him as well as anyone and I’m just so pleased to have him at the club.
“I spoke to Ryan Giggs to ask how much training he was doing and how fit he was and Ryan told me that, if he’d been manager at Old Trafford this season, he would’ve still been his No.1 centre-half.
“Rio was fit all last season, David Moyes changed the team around and didn’t pick him.”
Redknapp also reiterated his hope that Ferdinand’s presence at Loftus Road will persuade Steven Caulker to join after QPR agreed a fee with Cardiff City for the defender.
The QPR boss added “Rio has been the key. He would like to play with Rio and feels it’ll help him get back into the England team, which he is capable of.“The fee was agreed two weeks back, now it’ll hopefully get over the line."
Redknapp also confirmed that former West Ham midfielder Jack Collison is training with the club, while Esteban Granero has returned to pre-season fit after spending an injury-hit campaign on loan at Real Sociedad.
Speaking about QPR’s aims for the coming campaign, Redknapp recognises Premier League survival is a must and the signing of more players is his first priority.
“Tony Fernandes knows what we need and I'm sure we'll get deals done. We're in the market for players. We need to bring in five or six. We need to improve the squad in terms of quality and numbers,” he said.
“Staying up in the Premier League is our target this year. It's an incredibly tough league, but we're looking forward to it.”