All this talk about positional upgrades and Barca emulation, most people forget that we did quite well this season up until December, when it started falling apart. Our attacking play, even against the bottom sides, wasn't such a debacle until the turn of the year.
With Coutinho injured and Mane off to AFCON, I think Klopp made the mistake of trying to rectify a problem my making it into two problems. By moving Lallana up into the top 3 instead of slotting in an Origi for example into that hole, he put us into a position where the midfield lost it's effectiveness, and Lallana, who had played poorly up front in his Liverpool career, into a funk. Which was inexcusable up to that point as he was on a tear playing in midfield.
Then with six days between the Sunderland match and the FA Cup tie with Plymouth, he played the reserve side instead of the first string, who would have to wait until the Southampton cup tie in order to play again, which was a 10 day break. This in my opinion did two things, one was to break the team's rhythm, and two it meant that they went into the EFL Cup tie with Manchester United on their minds three days later. The latter meant that the team were probably weary of fatigue, and didn't want to expend too much energy dealing with a Southampton side that were in a lull at that time. This I think broke the team's momentum. And the most important of all, it dented their confidence. Because, with the draw with Sunderland, Plymouth and the loss to Southampton, the familiar refrain of us not performing against lower sides became a self fulfilling prophecy.
But honestly, looking back at it all, I think Klopp was to blame largely for what happened, because I think he has a very positive mentality towards football in general. This is good if his players had the same mental fortitude as he does, but in my mind I think most of our players are severely lacking in that area. His thought throughout, and it reflected in his interviews, is that the players should be strong enough to handle these situations. But either he wasn't aware of their mental fragility, or he wasn't able to influence them in the way that he wanted to.
In any case, if we were to address any sort of hole in our team in the summer, we need to buy players who are (in addition to technical and physical attributes) intelligent, humble, and most of all, strong leaders in their own right. The former I believe we lack in our squad at the moment (Moreno, Can, and Mignolet, no matter how smart Klopp says he is), the latter we just don't have much at all to be honest.