I really think it's hilarious that your guy doing the predictions for the end of the season thinks the Magpies are suffering solely because they hired a Chaser who hasn't played for six years. I mean, really, look at the scores and you'll see that there is a very different story.
Johnson may have been rusty the first game, and maybe she's on the injured list now, but the real downhill slide began because of Rodric and his point shaving shenanigans. Rodric slowly and systematically weakened the offense for how long before someone finally caught him throwing matches? As a former Gryffindor Captain, I know that half the success of any great team is spirit and team work. Rodric shattered the trust of his team mates.
Now look at the scores. Johnson is the top scorer for the Magpies. Her and Adler are the only players even trying anymore (and they're both on the injured list, so sorry Magpies fans, you're hosed). The other two buffoons aren't even trying. They certainly aren't passing. They're afraid to trust anyone on their team with the Quaffle. They're afraid to trust their beaters to get their backs. Adler probably has it easier in that regard, as he doesn't touch the Quaffle. He's only responsible for himself.
So instead of blaming Johnson's injury and her lack of experience, blame Rodric. Him and those other three twats soiled the game of Quidditch and should be drawn and quartered. They also should be banned from Quidditch for life. I realize that is a more reasonable option.