Jacksonville Jaguars running back Leonard Fournette was so rattled during the loss to the Tennessee Titans on Thursday, he got into it with a fan ... and threatened to beat the guy's ass.
TMZ Sports has footage of Fournette on the sidelines in the 4th quarter of the Jags' 30-9 loss at Nissan Stadium in Nashville ... where the RB angrily tells the fan, "Listen, you're too old for that. I will beat your ass!"
Two men who appear to be Jaguars staffers grab Leonard and try to walk him away -- but minutes later, Fournette begins to jaw at the fan again .