'That's For Me to Know, But I'm Healthy'

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NBA star De'Aaron Fox is shedding some light on why he parted ways with Team USA -- telling TMZ Sports it's NOT because he's injured!!!

The 21-year-old withdrew from Team USA earlier this month -- joining guys like Trae Young and Kyle Lowry who also recently backed out.

There have been questions surrounding Fox's move ... some people think he was nursing a secret injury, others say he wants to focus on the next NBA season. There's even a theory he was cut.

So, when we saw the Sacramento Kings star at LAX, we had to ask for the REAL reason he left.

"That's for me to know," Fox coyly told us ... adding, "I'm completely healthy."

So, the mystery continues ...