Nate Diaz ain't the only member of his family who thinks he cam beat down Conor McGregor ... his bro Nick Diaz tells TMZ Sports, "I'll whoop his ass!"
The elder Diaz bro was leaving Warwick with his pal, Bellator fighter AJ Agazarm when we brought up the possible trilogy fight between Nate and Conor ... a fight EVERYONE wants to see!
"I'll whoop his ass," Nick told us ... which triggered his pal A.J. to start talking a TON of crap about the Irishman.
"I can't believe he hit that old man!