The NHL has launched an investigation into Morgan Rielly -- after a hot mic seems to have captured the Maple Leafs star calling a referee a "f*cking f***ot" ... but there's a twist.
It all went down during Monday's Leafs vs. Tampa Bay Lightning game -- when Rielly got into a heated exchange with a ref ... and people watching on TV claim they heard the homophobic slur.
There's a debate over whether Rielly used the homophobic f-word or if he said, "F*cking rag it" ... a hockey term that essentially means to freeze the puck and waste time.