Lampard confirmed on Monday evening that he will quit the Blues this summer, with the 35-year-old widely tipped to join Sheikh Mansour's new Major League Soccer franchise New York City FC.
And QPR boss Redknapp, speaking at a press conference on Monday morning, felt the Blues may rue deciding to let his nephew go.
Absolutely I would keep him at Chelsea," Redknapp said of Lampard, who joined the west London club from West Ham in 2001.
"He's worth his weight in gold just to be around the place.
"I think Roy (Hodgson) feels the same. You need good characters around whether it is England or your football club.
"You've got (Steven) Gerrard and Lampard with England - proper guys that the youngsters can look up to."
The MLS season does not start until March, so 103-cap England midfielder Lampard could seek a short-term Barclays Premier League loan, with Redknapp's newly-promoted QPR among the potential suitors.
Lampard claimed 11 major trophies during his time at Stamford Bridge, making 648 appearances and becoming the club's all-time leading goalscorer.
Redknapp feels Lampard may have decided to chase a new challenge while he is still just fresh enough to offer a major impact at a new club.
"Maybe if he's going to go, now's the time," the former Tottenham boss said.
"I'm sure he'll be focused on the World Cup now - he won't be thinking about much else.
"It could be exciting to go to New York, though - a new project over there - but he'll be successful in whatever he wants to do.
"The only person Frank would ask for advice is his dad, but he'll know, he's a very shrewd boy.
"He's got his girlfriend as well, and they might fancy New York for a couple of years.
"I don't think Frank will be happy not playing regularly - he's not used to it and he won't fancy not playing."
Redknapp, who spent three years at the Seattle Sounders between 1976 and 1979, believes MLS still has some way to go to rival the calibre of stars that made the move some 40 years ago.
"I went out to Seattle and it's a great time, a fantastic experience," he said.
"Thierry Henry's out there now and Jermain Defoe, but in the 1970s there were much better players out there!
"Seriously, though, you had Franz Beckenbauer, Johan Cruyff, Gerd Muller, Pele and Carlos Alberto.
"Most of that great Dutch team of the 1970s went out there, and even Bobby Moore and George Best at points too.
"I think it was far more star-studded then, but it has moved on a level again recently."
Spain striker David Villa, 32, was confirmed as the New York franchise's first-ever player on Monday.
- Sports & Recreation
- Major League Soccer
- Sheikh Mansour