The 32-year-old completed a dream move back to Merseyside on Monday, having been released from Anfield as a youngster.
After five prolific years at the Saints, where Lambert's goals helped the south-coast club climb from League One to the top half of the Premier League, the England international joined his boyhood club for a reported €5.5 million [£4.5m] fee.
And former Liverpool winger McManaman thinks Brendan Rodgers has sealed a fine piece of business in capturing the striker.
"If he can replicate those kind of [goalscoring] figures for Liverpool I think they have got themselves a really shrewd bargain," he told The Mirror.
"As we know, football isn't about four-year signings or five-year signings, it's about next year.
"It's year to year unfortunately. It's a wonderful opportunity for Rickie."
The transfer represents a significant profit on the €1.2m [£1m] the Saints paid Bristol Rovers to sign Lambert in 2009, a fee he quickly repaid by guiding Southampton back to the top flight.
Lambert netted 106 goals in 207 league games during his time at St Mary's, and easily made the step up to the Premier League, scoring 15 times in his maiden campaign - a tally which made him the joint top-scoring Englishman that year.
Former Anfield captain Jamie Carragher has also praised the move, stating: "I think it's a very good signing. He's a local boy, a Liverpool fan, and it's probably been a dream for him."