The other day, some dude in a gas station parking lot threatened to kick my ass because he didn't like that I was over the line one side - no joke. Am I really allowed to sue him for that? Apparently, I should have taken down his information. Instead, I just got in my car really fast and got the heck out of there!
The legal answer: you can sue, and you might even win. The real question is damages. What injury did you sustain, and how do you quantify that? If you can no longer drive your car and have to go to twice-weekly therapy, you may have a case.
My guess is that the fan here is seeking damages due to defamation, rather than injuries related to the verbal threats. To win that case, though, you'd have to show that not only did the fan not say what Westbrook said he did, but that Westbrook knew or should have known that.
The case isn't going anywhere, because it's essentially impossible to prove either way what was said.