The UCLA basketball star went under the knife early Thursday morning to correct an irregularity that team doctors had spotted during a routine check-up earlier this year.

The good news ... Shaquille O'Neal's 18-year-old was in great spirits when he got out of surgery Thursday evening -- and posted a message to his fans (that he dictated to his younger sister).

"I made it, Thank you for all the love and support! I really appreciate all your thoughts and prayers, that's what helped me through this surgery."

He added, "I'm on the road to recovery, and I'll be back soon ... better than ever."

Shareef is already getting support from his famous friends including Dwyane Wade who wrote -- "Love you lil bro. You will come back stronger."

Austin Rivers added, "Stay up bro! Coming back stronger than ever."