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UCLA basketball star Shareef O'Neal -- Shaq's son -- says he'll be forced to sit out next season due to a heart issue that will require major surgery ... but vows to return ASAP.

TMZ Sports spoke with the 18-year-old superstar who said he was having a routine checkup recently when the docs "found a medical issue dealing with my heart."

Shareef didn't specify the exact problem -- but said the issue is severe enough that he'll be having heart surgery in the near future.

"Thank God the UCLA medical staff caught it early," O'Neal tells us.

Shareef says he's been talking with his dad and mom Shaunie O'Neal about how best to handle the situation -- and will NOT withdraw from school while recovering.

"During rehab, I'll be attending my classes and being a normal student."

Shareef says he vows to return to the court in 2019 after redshirting this upcoming season -- and describes the ordeal as "just a bump in the road."

"I'll be back in no time."

Shareef was one of the top high school players in the country in 2018 and was expected to have a real impact with UCLA this upcoming season.