Source: The Wrestling Observer Newsletter
As previously reported (and well-documented since he began), the original Sin Cara has had a rough time since beginning in WWE. Mistico, as he is known in Mexico, has had several injuries, in-ring problems and a Wellness Policy violation and Hunico has taken up the mask, picking up wins over the last few weeks.
As of now, Hunico is expected to stay working under the mask and that Mistico will be quietly leaving once his contract expires on January 10th. The reason they are keeping the Sin Cara character around is because his merchandising sales (masks, etc.) are through the roof.