It's easy, compare the two variables being discussed assuming that your teammates are pulled in a truly randomized system from the available pool of players in your MMR range. For the first scenario, what percentage of that player population is skilled enough to attain average or greater competency with a new champion with less than two weeks to study and practice with them (keeping in mind the probability of someone else doing the same first and producing online guides or tips/tricks to reference)? Now the second scenario; what percentage of that player population possesses average or greater competency with ANY existing champion in the same class as the new champion?
Assuming an equal Poisson distribution of player skill without any bizarre outliers, the latter scenario will have a probability of 50%. Which means that for **YumaS2Astral's **statement (that players using new champions are more likely to be bad with them) to be invalid, the probability of the former scenario would have to be equal to or greater than 50%.
I don't have the exact numbers for the probability that a player without any external help could reach average or greater competency with a champion in less than two weeks. But for the tiers that comprise the overwhelming majority of the playerbase, I do NOT see that scenario as being particularly common.
Even if you compare the first scenario to the probability of a third scenario - a player who is skilled enough to attain average or greater competency with an old champion with less than two weeks to study and practice with them - the player with the new champion still suffers, as Player #3 has access to online guides, tutorials, and gameplay/build data accumulated over that champion's lifespan. Player #1 does not. The only time that the odds of Scenario #1 match Scenario #3 is if the player in question has either deliberately or unintentionally ignored or overlooked every piece of advice and help with regards to the old champion that they have just started playing.
And it is gambler's fallacy. Look at each individual player on a team as a separate event instead of each game as a separate event, and use the odds of getting a lousy player. Your skill level has no effect on the probability of each individual player on your team being skilled; if anything, since teams are matched against each other by the average MMR of all the team's players, if you are a skilled player then you are more likely to be paired with a bad player than if you were merely average (assuming individual MMR to be a generally accurate representation of skill).