No, the solution is not superglue 
The referee should carry a spare mouthpiece for each fighter. If the mouthpiece comes out, the referee puts in the clean one out of his pocket, kicks the dirty one out of the ring and the fight goes on with almost no delay. Heck, carry two spares for each fighter and maybe Castillo would be the defending champ for the rematch.
Hopefully the rematch doesn't have any controversial moments, but I haven't heard of anything done to anticipate another mouthpiece delay strategy.

The referee should carry a spare mouthpiece for each fighter. If the mouthpiece comes out, the referee puts in the clean one out of his pocket, kicks the dirty one out of the ring and the fight goes on with almost no delay. Heck, carry two spares for each fighter and maybe Castillo would be the defending champ for the rematch.
Hopefully the rematch doesn't have any controversial moments, but I haven't heard of anything done to anticipate another mouthpiece delay strategy.
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