I construct my views on the matter considering the entire time Postecoglou has been the manager, I won't flip because of one particular match, like you said you would.
Johnson's goal was a scuff of the side of the ball that trickled into the goal, he didn't place the ball with any accuracy or blast it into the net, he was lucky the way it bounced off of Luke Shaw, the game could have easily have gone the other way.
You're going on like something amazing happened, and it's caused you to change your opinion about the manager. We've beaten Bodo and the worst Manchester United side in living memory. The fact that both United and ourselves even reached the final just goes to show how weak the competition was. The only decent side we had to beat was Frankfurt to reach the final.