The coach who fought Diddy this week ... brutally shamed Diddy's son by using a paparazzi pic to drive home a point -- that he thinks Justin Combs is a spoiled rich kid.
Sources connected to UCLA's football program tell TMZ Sports ... the humiliating incident went down in February. Coach Sal Alosi had several players in the the gym for a training sesh, which Justin missed because he was at the NBA All-Star game.
We're told Alosi openly mocked Justin by putting a pic of him at the game -- next to Diddy and Ben Stiller ... in all the gym monitors. We're told Alosi told players ... "While you guys are training, this guy is sitting courtside with his dad." The pic remained up for 24 hours.
As we've reported ... Diddy sources say Alosi ran a campaign of psychological warfare against Combs for the past 3 years.
Our UCLA sources say over the years, Alosi has told Justin things like -- "You're only here because of your dad," and ... "You never belonged here," and ... "You should just go work for your dad."
We reached out to Alosi, through UCLA ... so far, no response.