To say that some controversy has followed Rey Mysterio since the time he left the WWE a few months ago would probably be considered a pretty big understatement. Though Rey's full-time in ring career may likely be over at this point he has still been apart of the same company for the last 10 years and is considered a very valuable commodity for those who can afford to pay him.
Lucha Underground has been somewhat thrilling Independent wrestling fans on and off since it's debut on the El Rey Network a few months ago. Now it might be fair to say that it took the American fans sometime to get used to the style that was seen weekly on Lucha Underground, but once they warmed up to it most fans have loved the show. Now take into account a Mexican Wrestling legend that is free to be apart of any company he wants to for the first time in 10 years. Right now I would think that calling Rey a Semi-Active or Part-Time wrestler wouldn't be to far from the truth, and even though that is true Rey still has tremendous drawing power among some fans who haven't seen him in the ring for 10 years or even longer in some cases.
Though at this point Lucha Underground's first season is drawing to a close and the likely hood that Rey will show before the end of the first season being very slim. I would figure that Rey would be a major factor and a big selling point for Lucha Underground going into Season 2 of the show.