According to PWMania.com:
Last night at the Monday Night RAW/Friday Night Smackdown supershow double taping in St. Louis, it is believed that Shawn Michaels suffered a triceps tear during an in ring confrontation with Chris Jericho.
PWMania posted this quote from Kyle Hartnett, who attended the tapings, just a few minutes ago:
"As it was coming to a close, Lance Cade came through the crowd and attacked Michaels. HBK quickly gained the upper hand as he was laying punches down on Cade. Jericho tried grabbing him from the outside, and that's where HBK took an awkward fall going through the ropes down to the mat, nearly landing on his face, but was able to brace his fall at the last second.
"Jericho then left after a brief staredown. Then HBK delivered a few more kicks to Cade's face as the show ended."
It is being reported by Steve Gerweck that Shawn does intend to work at this sunday's Unforgiven pay per view.
If the injury really is a triceps tear, than it will almost certainly require surgery which will put HBK out of action around six months.
I will post more updates throughout the day as they become available