Booker and the Suns -- who played all five starters -- lost to the Adelaide 36ers, 134-124, in their first preseason game ... the first time an NBA team lost to a National Basketball League team.

After the L, Book posted a screenshot of LeBron's viral speech following the Heat's Finals loss ... adding the caption, "Mood."

Remember, LBJ was asked if he was bothered by fans rooting for his downfall ... and the King offered an honest, candid response -- but it was met with backlash for how it came across.

"At the end of the day, all the people that's rooting for me to fail, they have to wake up tomorrow and have the same life that they had before they woke up today," James said.

"They can get a few days or a few months on being happy about not only myself but the Miami Heat not accomplishing their goal, but they have to get back to the real world at some point."

James would clarify his comments a few days later ... stating he wasn't saying he was superior to others, but rather everyone has to move on with their lives, good or bad.