This is really unfortunate. A torn ACL is about as bad injury as you can have. It's an injury frequently suffered by professional football and basketball players, and suffered by an athlete it's almost always season ending and treated with surgery. According to what Andy says, they are trying conservative treatment, hoping, I'm sure, to get him back on stage (though limited) as soon as he can tolerate the pain.
I now see that he's planning to go on tonight so I'm guessing that the ACL tear is a partial tear and not a complete tear which is good news.