Shareef O'Neal Undergoing Heart Surgery

Shareef O'Neal is currently undergoing heart surgery to correct an irregularity that forced the UCLA basketball star to miss the 2018 season ... TMZ Sports has learned.

We broke the story ... Shaq's 18-year-old son -- one of the top college freshmen in the country -- learned about his medical condition during a routine checkup with team doctors earlier this year.

At the time, Reef told us, "Thank God the UCLA medical staff caught it early."

On Thursday morning, Shareef went under the knife -- and his mother, Shaunie, was there to see him off.

"This morning was by far my toughest moment being a Mom," she said in a statement.

"If only I could take his place today. Could I at least go with him in that operating room and hold his hand. Separating from him as he was wheeled off to surgery was heartbreaking. THATS MY BABY!"

"I don’t care how old, how big, how grown my kids get I still want to protect them. I had no control over any of this. But there is a God, I trust and believe He is in there with my baby right now and has complete control."

Shaunie is asking for prayers for Shareef.