The defence started to become shit when Rodgers moved Gerrard to the lone DM role. Obviously they scored a lot of goals since then, but without Suarez AND Sturridge they're not really that threatening offensively.
I'm actually quite optimistic they will start playing solid (at least) again when Sturridge finally comes back. I think the last game he played was against Tottenham, the only Liverpool performance that resembled last season.
While I agree that Gerrard at DM has some of the blame, I think most of it is on the manager. Rodgers claimed defending is easy. Made it sound like that's something any manager can teach a team. Last season, they conceded 50 goals in the league like some midtable team. Many of them were from set pieces and their problems with them are continuing. They can't mark their opponents even if their life depended on it. They would be a much better team if they tried to fix it, but it doesn't look like they are trying.
Sturridge would definitely improve them, until he gets injured again. Unfortunately for them, that's how it is with Sturridge. Injury prone as a Man Utd defender. On another note, they really wasted the Suárez money on shit. Rodgers really has no clue in the transfer market. The only decent transfers he's made so far are Coutinho and Lallana. The rest have been utter shit.