Welp, Aaron Carter's chances of beating Lamar Odom next month ain't looking great ...
Video of the pop singer getting destroyed by an amateur's body shot has just surfaced ... and, yeah, somewhere Odom's got to be smiling.
TMZ Sports has learned it all went down on May 4 ... when Carter was in a Santa Monica gym prepping for his June 11 Celebrity Boxing tilt with the ex-NBA star.
We're told Carter was mixing it up with Ian Feiner -- a fellow gym-goer who he had sparred with several times before -- when he got caught by a right hook to the gut.