Johnsons still had a bad game defensively, he's way to far up the pitch and easily passed, effective up front for sure, but its not because he's good, he's up against Ridgewell, whose having a bad game, which puts him at about the worst in the prem, on his best day he's not far off the worst in the prem.
Johnson's only "doing" him because Ridgewell is reacting first, the first time Ridgewell pre-emptively stepped to his left, giving Johnson tonnes of space inside, the next big chance Ridgewell ran out then did a little jump, he was out of control, going in one direction the second he jumped Johnson changed direction. I'll give him he's playing RIdgewell for the fool he is, and taking advantage of his horrendous blunders, but most would.
The thing of it is, Liverpool were comftable at 0-0 while Birmingham were happy with a draw. THe second they scored Birmingham actually pushed for a goal exposing every Liverpool weakness, defence basically, the second Birmingham tried Liverpool looked under pressure, which also meant their unpressured unhurried play out of defence went away so Liverpool recieved the ball upfield less.
When even Birmingham pushing forwards makes Liverpool look awful defensively you've got a problem, they've managed a total of 8 goals so far and already have two against Liverpool. Both were actually well made goals, and both showed how easy it was to get through the midfield and defence when they wanted to.
You get the feeling if Birmingham had pushed from the start they'd simply be further up and Liverpool wouldn't have gotten a goal.