Jurgen Klopp's side were beaten 3-0 by Watford on Sunday.
They were extremely disappointing against the in-form Hornets, with the newly promoted side dominating Klopp’s team on all fronts. And Barnes says that is because his old team are incapable of doing the ugly parts of the game such as physically out-muscling teams.
"For Liverpool to win they have to play good football, they have to play the way they played against Manchester City," he told Sky Sports.
"If they are not able to do that are they strong enough? Are they physical? Can they out-muscle teams? Can they match teams and they can't.
"I love Jurgen Klopp, he's a great manager. But at the start when he came I said to not get carried away, thinking all of a sudden things are going to happen. He has good players, don't get me wrong but to be where we want to be we need better players."
Barnes' comments are certainly justified in light of the loss to Watford, with Liverpool missing a competitive edge in that game.
That was most evident in midfield, with the likes of Jordan Henderson and Emre Can not looking like the type of competitive midfielder they are sorely missing.
Another concern will be their over reliance on Philippe Coutinho, with teams having figured out that negating his influence usually nullifies Liverpool in the process.
In both regards, Liverpool and Klopp must find an effective Plan B that can allow them to start getting wins under their belt once again.