Jurgen Klopp's team have come under fire numerous times for leaking goals, and this season so far they have conceded 18 in 10 matches in all competitions.
Klopp recently told the media that he could "write a book on defending in two hours", but Barton is not convinced by the club's options, which include Joel Matip, Dejan Lovren, Ragnar Klavan and Joe Gomez.
"Liverpool are great going forward, they've got a Champions League attack, but they've got a Championship defence," Barton told talkSPORT. "If you've going to build a side to challenge the big clubs in this league then you've got to be able to defend.
"Klopp's statement that he can write a book on defending... anyone can write a book on defending! We can sit here and go through footage and draw out how we'd perfectly like to defend a set-play - it's easy to do that - but defending is all about attitude and I don't think he's bought players who have the right attitude towards defending.
"You can say that it's individual mistakes, it's this, it's that, but they haven't got a desire to put their head in places and get hurt and defend properly. You've got to have a desire to keep clean sheets."
Liverpool currently sit fifth in the Premier League table.