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Reposting because I think it makes sense:
To everyone still doubting that the red shuriken is against Alan, I suggest you still quit while you're ahead. Today's episode showed that the field in that battle is absolutely the grass field from today's battle (take note of the shrub on the left and the sloping hill on the right) while Ash Greninja vs Mega Sceptile happens on a rock field that already got changed into from a forest field. They use 3D models for the battlefields so it's not an animation error, the field during Ash vs Shota is not going to change AGAIN towards the end of the last matchup and Ash vs Ayaka is in a townscape battlefield (and unlikely to change since it'll be a 3vs3). Unless you believe that the red shuriken is somehow from one of the other two matches that Ash will have next episode, it can only be against Alan.