Brendan Rodgers' side finished second in the Premier League last term, only losing the title to Manchester City after slipping up late.
But they have made a poor start to the current campaign, losing three of their first five matches to sit 11th in the table.
The close-season departure of star striker Luis Suarez – for a £65million fee to Barcelona – has hurt Liverpool and the thigh injury suffered by forward Daniel Sturridge has left them bereft of firepower, despite the arrival of Mario Balotelli from Milan.
Hamann, who made 283 appearances for the Anfield outfit from 1999 to 2006, feels that Suarez's stunning season last term, in which he scored 31 goals in 33 Premier League matches, masked Liverpool's problems in defence.
And he has called on them to adopt a new style if they are to improve.
"They concede too many goals," Hamann said.
"They conceded too many goals last season, but this season they haven't got the firepower, especially now with Sturridge being out injured.
"They have to adjust their game, they have to change their mentality, be a bit more defence-minded and try to keep the odd clean sheet, which they haven't really been doing for the past 12 months.
"I think the mindset has to change, because if you haven't got the firepower going forwards then you need to put a little bit more emphasis on the defence.
"I think this is where they have to change because even last year they conceded too many goals, which wasn't highlighted because they scored so many.
"But if they haven't got the firepower going forward this year then you'll be in trouble and they have to change quick."
Much has been made of the arrival of Balotelli and the controversial striker has made a mixed start – failing to score in three league games but netting a crucial UEFA Champions League goal against Ludogorets last week.
"I think he [Balotelli] has done all right," Hamann added.
"He scored a very good goal midweek and he's got to show now that he's a team player because the fans won't take anything less and neither will the manager.
"He's just got to play for the team, he's had his problems in the past but you've got to give him a chance.
"He won't get many more chances [but] so far, so good.
"But again you've got to take it week by week and game by game with him. I hope he sorts his act out because when he concentrates on his football he's a terrific player."
