In his suit, Keisel admits he tried to heckle Westbrook by telling the NBA star "that he should just sit down and ice his knees."
But, Keisel claims Westbrook misheard him believing he shouted a homophobic insult -- "get down on your knees like you used to."
Westbrook went nuclear on Keisel and his girlfriend, Jennifer Huff -- yelling, "I'll f*ck you up. You and your wife. I'll f*ck you up." Video of Westbrook's comments went viral.
In fact, Keisel says he was banned from the Vivint Smart Home Arena and all Utah Jazz games for life. Plus, Keisel says he was fired from his job, kicked out of a pilot training program and has received numerous death threats.
The Jazz claims they conducted an investigation before issuing the ban -- but Keisel says the probe is massively flawed ... with no video evidence showing him making any sexual or racial slurs. And, Keisel says witnesses who were sitting in the area are siding with him, not Westbrook.
He and Huff are now suing both Westbrook and the Jazz for $100 million for defamation, intentional infliction of emotional distress, false light and more.
A source close to Westbrook slammed the claims telling us, "The lawsuit is meritless. The Jazz conducted their own investigation of the guy's conduct at that game and based on their investigation, they decided to ban him from attending any future Jazz games."
"Westbrook's entire camp is confident the court will support the Jazz's findings."