The caution comes as Lampard prepares for his first season in charge having lost the services of star plater Eden Hazard to Real Madrid earlier this summer.
Though Christian Pulisic was signed in January and a permanent deal to sign loanee Mateo Kovacic was confirmed on July 1, Lampard is unable to make any further additions to his squad to the FIFA-imposed transfer ban that has been placed on the club.
He told Sky Sports : "The expectation will be to finish in the top four, but they've got to be realistic. I think there'll be moments in the season where it will be a struggle, because of the Hazard effect.
“It doesn’t matter what manager you have – [Pep] Guardiola, [Jose] Mourinho, anyone – they are going to struggle without that quality of player.
"But if they were to finish in the top four that would be an amazing season considering the transfer embargo and everything else.”
Chelsea finished third in the Premier League last season and won the Europa League but struggled for goals at times and will be unable to add to their firepower unless the ban is lifted.
Redknapp believes that patience, a quality not often on shown at Stamford Bridge in recent years, will be required as Lampard makes the big step up from managing one season in the Championship with Derby County.
“It’s a great move, not just for Frank but for Chelsea too,” he said. “It’s astute, you need someone who can stabilise the club somewhat when you’ve lost your best player and are under a transfer embargo.
“They’ve got one of their own, the fans adore him. He’ll need time, there’ll be a lot of changes; you can’t bring in any personnel to replace a big star name like Eden Hazard. He’ll need to be given time, but he will – because he’s Frank Lampard.”
Questions also remain about whether Lampard is being promoted too quickly having just one season as a manager under his belt, but Redknapp believes the timing is right given the circumstances.
“I know he enjoyed his time at Derby. He had a great rapport with the fans but you can’t turn something like this down.
“Everyone will have their opinions, some people will say it’s too early, others people will say it’s the right time, but for someone like Frank – he knows the game, he’s worked with some of the best managers in the world, and in terms of timing it is the right time.
“The players will respect him, and patience will be the key,” added Redknapp.