Sin Cara Tweeted on Thursday he's set to undergo a "mini surgery." The Tweet is below:
Thanks for all your support. On my road to the hospital for a mini surgery. See you later guys.
— Sin Cara WWEOfficial (@TheSinCaraWWE) December 20, 2012
I can't confirm what type of surgical procedure he is having but Sin Cara has been hampered by a legitimate left knee injury. The injury bothered him considerably at WWE TLC and the word making rounds after the show on Sunday was that he would need time off. Dot com did an injury angle with The Shield "taking out" Cara on Super Smackdown Live to write him off television.
You can read dot com's kayfabe update at this link.