Kevin Durant Says J.R. Smith's Bonehead Play 'Broke' The Cavs

J.R. Smith's Game 1 blunder wasn't just responsible for LeBron breaking his hand ... it was also the reason the Warriors broke the Cavs -- so says Kevin Durant.

K.D. went on "Jimmy Kimmel Live!" Monday night and explained to the talk-show host how J.R. brainfarting and running the wrong way at the end of regulation to start the series essentially ended the Cavs.

"We knew we kind of broke them a little bit with that one," the NBA Finals MVP said.

There's more ... Durant says there will be no summer of drama when it comes to his Bay Area future -- 'cause he ain't about to leave the Dubs' dynasty yet.

As for if LeBron wanted to join up with him, Steph, Klay and Draymond -- K.D. didn't seem so sure about that.