The Reds are looking to beef up their squad with sufficient quality ahead of a new season that includes a first Champions League campaign in five years.
The five-time European Cup winners have already landed striker Rickie Lambert from Southampton for around £4 million, and they have been linked to a whole host of players including Bayer Leverkusen midfielder Emre Can and Sevilla left-back Alberto Moreno.
McManaman, who played for the club during the 1990s before leaving for Real Madrid, thinks that manager Brendan Rodgers may need as many as half-a-dozen signings in the next transfer window.
Speaking to the Liverpool Echo, the BT Sport pundit said: “I think Liverpool need to bulk up in all areas. If they can bring five or six players in who will improve their team and improve their squad, if you’re a Liverpool fan, that’s what you want.
“Liverpool will need a significant number of players if they are to compete in the league and the Champions League. I like the spine of the team to improve. If they can improve on those areas, I think they will be great. Steven Gerrard is not going to play forever. Joe Allen and Jordan Henderson have done well this year, but you can always improve can't you.
“If you can bring in another centre forward, you will make the fans very happy. It’d be nice if you can get English players but unfortunately English players now cost double what everybody else does abroad."
The Reds are interested in Southampton playmaker Adam Lallana,and their pursuit was reportedly stepped up last week when they offered a 'take or leave it' offer of around £25 million for the Saints skipper.
Ashley Cole is available on a free transfer after his Chelsea contract expired and the former England international is another possible alternative to the left-back spot.